CSS: Setting for not validating vendor-specifics

Implements the feature request from
http://webessentials.uservoice.com/forums/140520-general/suggestions/6190962-add-option-to-disable-browser-compatibility-warnin
CSS: Added setting for duplicate selector

Implements the suggestion from
http://webessentials.uservoice.com/forums/140520-general/suggestions/6674182-add-option-to-disable-duplicate-selectors-warnings
JavaScript: Cleaned up comment completion
CSS: Clean up
JS: Added @callback to JSDoc completion
Project: Set Copy Local to false for non-GAC VS assemblies
Project: Fix editor settings files

Add Rebracer settings
Add some more editorconfig file types
Markdown: Start porting to Roslyn

Doesn't quite work yet
Project: Remove old NuGet Package Restore setup

This NuGet version was too old for some of the Roslyn packages.
And VS now automatically restores packages on build, so it isn't
necessary in the first place.
Markdown: Fix Workspace setup for diagnostics
Markdown: Add documentation provider

Thanks, @JasonMalinowski!
Markdown: Install Roslyn command target