Basics of new override ability by extension, which will also allow other file extensions to be formatted.

[belav]

closes #1220

I mostly went with how prettier works for overrides. Using a file glob, a user can specify options to be used for a file. One of those options is which formatter to use. Right now this means non-standard files can be formatted with the csharp formatter. Some day the xml formatting PR will finally get some attention.