Adds between' combinator which acts like between but doesn't consume …

[timmytofu]

Currently:

> parseOnly stringValue "\"   hi\""
Right (StringValue "hi")

With between' (not the best name, maybe):

> parseOnly stringValue "\"   hi\""
Right (StringValue "   hi")

it properly captures the leading whitespace in string. It still eats whitespace after the closing quote, though, to follow the token system.

[jdnavarro]

String literals need to be actually taken care of, right now the stringValue combinator parser is just a stub. If you are feeling adventurous you can try to include a proper parser with this PR (or a new one if you prefer). Here are some source you can look up for hints:

• Easy to implement but may be slower: https://hackage.haskell.org/package/parsers-0.12.2.1/docs/Text-Parser-Token.html#v:stringLiteral
• May be faster but more obfuscated: https://hackage.haskell.org/package/aeson-0.10.0.0/docs/Data-Aeson-Parser.html#v:jsstring

It's not my intention to provide parsing combinators with this package. The current parsing combinators are meant to be used internally only, but I still have to make them private.

[timmytofu]

I saw the TODO about escapes, I will see if I have time to work on it.