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.
I thought about that, but was scared to muck with something that clearly has different expectations on what is and is not considered a `name`.  For instance, you can apparently pass a symbol to software_specs' option, which isn't quite what that regex is built for.  I'll try it out and see what happens.