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.
You'd use the scope resolution operator instead of `.` to access it:

```ruby
class Option
  OPTION_NAME_REGEX = /^(?:--)?([^=]+=?)(.+)?$/
end

Option::OPTION_NAME_REGEX #=> /^(?:--)?([^=]+=?)(.+)?$/
```

(But I'd also prefer Jack's suggested approach!)