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 think it would be cleaner to do this normalization in https://github.com/Homebrew/homebrew/blob/56b57d583e874e6dfe7a417d329a147e4d4b064f/Library/Homebrew/software_spec.rb#L101 rather than here or in the DSL method below.