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_mcs_command_callbacks.vader

@@ -0,0 +1,34 @@
+Before:
+  Save g:ale_cs_mcs_options
+
+  unlet! g:ale_cs_mcs_options
+
+  runtime ale_linters/cs/mcs.vim
+
+  let b:command_tail = ' -unsafe --parse'
+
+After:
+  Restore
+  unlet! b:command_tail
+  unlet! b:ale_cs_mcs_options
+  call ale#linter#Reset()
+
+Execute(Check for proper default command):
+
+  let b:command = ale_linters#cs#mcs#GetCommand(bufnr(''))
+  let b:command = substitute(b:command,'\s\+',' ','g')
+
+  AssertEqual
+  \  b:command,
+  \  'mcs -unsafe --parse %t'
+
+Execute(The options should be be used in the command):
+
+  let b:ale_cs_mcs_options = '-pkg:dotnet'
+  let b:command = ale_linters#cs#mcs#GetCommand(bufnr(''))
+  let b:command = substitute(b:command,'\s\+',' ','g')
+
+  AssertEqual
+  \ b:command,
+  \ 'mcs' . b:command_tail . ' ' . b:ale_cs_mcs_options . ' %t',
+