Allow custom predicate that replaces StringReader#isAllowedInUnquotedString

[astro-angelfish]

Instead of #131 with greedy charset solution, I tried another approach that allows users to define their own whitelist. It might be useful when users trying to define their own syntax on command arguments like entity selectors or something else that might accept non-ASCII characters

Usage of StringReader#readString or StringReader#readUnquotedString without any argument would lead to the default behavior, which is calling StringReader#isAllowedInUnquotedString as their whitelist predicate.

Fix #103.

[Privatech38]

This will cause problems when dealing with larger whitelists/list of allowed chars, therefor consider merging yours and #131's way of dealing with this issue.