Added consume_definition

[That3Percent]

This PR adds a new API consume_query:

/// Parses a single ExecutableDefinition and returns an AST as well as the
/// remainder of the input which is unparsed
pub fn consume_definition<'a, S>(s: &'a str) -> Result<(Definition<'a, S>, &'a str), ParseError> where S: Text<'a> { ... }

There are 4 tests demonstrating the functionality.

Resolves #35
Replaces #36

[tailhook]

Merged! Sorry for the long delay.

[That3Percent]

No problem! Thanks so much.