Implemented review recommendations

Implements suggestions and recommendations suggested by the first review
of the "Advance C# linter based on mcs -t:module (#952)" pull request.

- Clarifies and simplifies description of linters and options
- Added links to help file and marked the mcsc linter as to be run only
  when file in buffer is saved or loaded.
- Added comments to the mcsc.vim file to clarify code
- removed type checks considered not necessary be reviewer.
- addresses findings by vader
- removed call to getcwd and cd in vim script
- handler expands file names relative to route of source tree into
  absolute pathes. Fixes errors not being marked when vim is started
  from subdirectory of source tree.
- implements tests for mcs.vim and mcsc.vim linter

test/command_callback/test_cs_mcsc_command_callbacks.vader

@@ -0,0 +1,120 @@
+Before:
+  Save g:ale_cs_mcsc_options
+  Save g:ale_cs_mcsc_source
+  Save g:ale_cs_mcsc_assembly_path
+  Save g:ale_cs_mcsc_assemblies
+
+  unlet! g:ale_cs_mcsc_options
+  unlet! g:ale_cs_mcsc_source
+  unlet! g:ale_cs_mcsc_assembly_path
+  unlet! g:ale_cs_mcsc_assemblies
+
+  let g:temppath = fnamemodify(tempname(), ':p:h')
+  let g:temppathpattern = substitute(escape(g:temppath, '\\/.*$^~[]'), '[\\/]*$', '[\\\\/]\\+\\S\\+','')
+  let g:sometempfile = fnamemodify(g:temppath .'/some_temp_file.tmp', ':p')
+
+  runtime ale_linters/cs/mcsc.vim
+
+After:
+  Restore
+  unlet! b:ale_cs_mcsc_options
+  unlet! g:ale_cs_mcsc_source
+  unlet! g:ale_cs_mcsc_assembly_path
+  unlet! g:ale_cs_mcsc_assemblies
+  unlet! g:temppath
+  unlet! g:temppathpattern
+  unlet! g:sometempfile
+  call ale#linter#Reset()
+
+Execute(Check for proper default command):
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command, '\s\+', ' ', 'g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+Execute(The options should be be used in the command):
+
+  let g:ale_cs_mcsc_options = '-pkg:dotnet'
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command, '\s\+', ' ', 'g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe ' . g:ale_cs_mcsc_options . ' -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+Execute(The souce path should be be used in the command):
+  call ale#engine#Cleanup(bufnr(''))
+  call ale#engine#InitBufferInfo(bufnr(''))
+
+  let g:ale_cs_mcsc_source='../foo/bar'
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command, '\s\+', ' ', 'g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd "' . g:ale_cs_mcsc_source . '";mcs -unsafe -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+Execute(The list of search pathes for assemblies should be be used in the command if not empty):
+  call ale#engine#Cleanup(bufnr(''))
+  call ale#engine#InitBufferInfo(bufnr(''))
+
+  let g:ale_cs_mcsc_assembly_path = [
+  \ '/usr/lib/mono',
+  \ '../foo/bar'
+  \]
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command, '\s\+', ' ', 'g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe -lib:"' . join(g:ale_cs_mcsc_assembly_path,'","') . '" -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+  let g:ale_cs_mcsc_assembly_path = [
+  \]
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command, '\s\+', ' ', 'g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+Execute(The list of assemblies should be be used in the command if not empty):
+  call ale#engine#Cleanup(bufnr(''))
+  call ale#engine#InitBufferInfo(bufnr(''))
+
+  let g:ale_cs_mcsc_assemblies = [
+  \ 'foo.dll',
+  \ 'bar.dll'
+  \]
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command,'\s\+',' ','g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe -r:"' . join(g:ale_cs_mcsc_assemblies,'","') . '" -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+
+  let g:ale_cs_mcsc_assemblies = [
+  \]
+
+  let b:command = ale_linters#cs#mcsc#GetCommand(bufnr(''))
+  let b:command = substitute(b:command, '-out:' . g:temppathpattern, '-out:' . g:sometempfile, '')
+  let b:command = substitute(b:command,'\s\+',' ','g')
+
+  AssertEqual
+  \   b:command,
+  \   'cd ".";mcs -unsafe -out:' . g:sometempfile . ' -t:module -recurse:"*.cs"'
+