Add OPTION_NAME_REGEX, a standardized method of parsing option names.

Allows putting example option values in option names to make `brew options formula` as useful as possible.  See [this comment](https://github.com/Homebrew/homebrew/commit/d21ccee108fc56d56f912b10e48969c373356cb9#commitcomment-8843913) for more details.
As written, this could just be a constant, though I think I'd rather see a class method like `Option.normalize_name(string)` rather than exposing a constant regexp in this way.