(Re-open of #55 using another branch)

The spec says type flag for some tokens, while this module outputs isInteger, isIdent, etc. instead, which is not consistent with it.

Unfortunately the type property is already used to determine the token/rule type. A simple workaround is to use TYPE instead, as this special property is mostly used for the JSON output (in other context we can simply check by obj instanceof theTokenClass or obj.constructor.name === theTokenClass.name instead).