Add a coverage metric

[jdkato]

It's not completely clear to me how to go about doing this, but I'd like to have some way of measuring how much of a style guide is covered -- e.g., "We implement 80% of the Microsoft's guidelines."

My initial plan is to do this on a per-file basis using the source of the guide (https://github.com/MicrosoftDocs/microsoft-style-guide).