Make extension methods ignore drives

In the Windows version of the API, `"//foo" <.> "txt" == "//foo\\.txt"`. That's not what anyone expects, and doesn't match the .NET conventions for filepaths. It also means Shake can't use the `<.>` FilePath operator with its file globbing syntax as per https://github.com/ndmitchell/shake/issues/198 (which isn't super important, but does hint it might be desirable to change).

My thoughts are the extension of a filepath should be the bit after both the final `pathSeparator` and final `.` character. That's a simple definition, consistent with the Posix version, and only changes things in the not-a-complete-UNC-path case, which isn't really a valid FilePath anyway. Plus it probably matches what everyone thinks the operator does anyway.

I have a slight concern here that I might be biased towards doing something that makes it easier for Shake, but is ultimately the wrong thing to do, since I've spent a lot of time on Shake. If anyone else thinks I'm wrong, or even right, please yell.