Use a single token for identifiers

https://github.com/Shopify/liquid/pull/463 separated identifiers into multiple tokens, but failed to take into account some edge cases, such as `foo-2` in which the 2 would be lexed as a number. Having a single token keeps this simpler, and I set up the regex to still ensure question marks may only appear at the end of the identifier.

@fw42 @xthexder