Allow `--compilers` to specify plugins without an extension

I'm creating a little module that adds syntactic sugar to test declarations, defining test levels ("smoke", "full" etc) inline in the code, and skips with reasons (broken, todo, etc).
One nice way to load this would be to use the existing `--compilers` flag in mocha.
I can specify `--compilers=:mymodule` and it works great, but it appends that empty string to the regex pattern of extensions to look for.
I could put a bogus extension in there like `--compilers=zzz:mymodule`, but that seems silly.
Using `--compilers=plugin` to load plugins between Mocha and test files seems like a nice elegant solution.
Thank you for considering this PR and for all your great work on Mocha.