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
