Implement quoted string unescaping

[null-dev]

Fixes: https://github.com/mockersf/hocon.rs/issues/53

Partially based on: https://github.com/changhe3/nom-json-parser/blob/cce34f560f394cdf1601ab9c5c2fd012eb405084/src/utils.rs#L73

Also, I'm not really familiar with nom so I'm not sure if this is the best place to hook in the unescape function. Maybe look over that if possible.

This PR also adds two dependencies but hocon.rs already depends on them transitively:

• aho-corasick - Previous transitive dependency chain: hocon.rs -> java-properties -> regex -> aho-corasick
• lazy_static - Previous transitive dependency chain: hocon.rs -> java-properties -> lazy_static

[mockersf]

seems good to me, thanks!