Alllow specific files to be excluded, not just file extensions

[deadlydog]

From a comment on the VS 2017 extension reviews:

Ability to provide full stops in file extensions to exclude would help massively (excluding feature.cs files yet still including any .cs files)

I'm thinking it would probably be best to allow users to define a (or many) regular expression which determines the files to exclude from being diff'd. That would offer the most flexibility.