Add simple character based property test

[lukaszsamson]

This PR adds property tests that generate random strings in various elixir contexts.
Assumptions:
in tolerant mode

1. if original parser returns ok, spitfire has to return exactly the same AST
2. if the original parser crashes or errors, spitfire may not crash
   in strict mode
3. if original parser returns ok, spitfire has to return exactly the same AST
4. if the original parser crashes or errors skip spitfire and assume the case passed