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

doc/ale.txt

@@ -36,6 +36,9 @@ CONTENTS                                                         *ale-contents*
       cppcheck............................|ale-cpp-cppcheck|
       cpplint.............................|ale-cpp-cpplint|
       gcc.................................|ale-cpp-gcc|
+    c#....................................|ale-cs-options|
+      mcs.................................|ale-cs-mcs|
+      mcsc................................|ale-cs-mcsc|
     css...................................|ale-css-options|
       prettier............................|ale-css-prettier|
       stylelint...........................|ale-css-stylelint|
@@ -222,7 +225,7 @@ Notes:
 * C: `cppcheck`, `cpplint`!!, `gcc`, `clang`, `clangtidy`!!, `clang-format`
 * C++ (filetype cpp): `clang`, `clangcheck`!!, `clangtidy`!!, `cppcheck`, `cpplint`!!, `gcc`, `clang-format`
 * CUDA: `nvcc`!!
-* C#: `mcs`
+* C#: `mcs`, `mcsc`!!
 * Chef: `foodcritic`
 * CMake: `cmakelint`
 * CoffeeScript: `coffee`, `coffeelint`