Make identifier chars configurable

Some languages allow more non-alphabetic characters in identifiers than just `'_'`. This pull request allows the user to configure which non-alphabetic chars should be accepted as part of identifiers.

For example for clojure development I set this to `'_-/.><'`.

I went for the simplest solution here, but it might be worth it to make it a regex. I can make that change if it's preferred.