fix renaming .coffee.md to .js

This commit fixes the behavior of renaming `foo.coffee.md` to `foo.coffee.js`. The `coffee.md` extension is automatically detected but not transformed properly.

This is a bit brute force, as it just transforms `.coffee.md` to `.litcoffee` before passing it off to `gutil.replaceExtension`, but this doesn't require changing `gutil`'s reasonable behavior in assuming that `.coffee` shouldn't be removed.