Allow custom primitive parsers

Adds `Parsimmon.custom` to allow the definition of primitive custom parsers.
Custom primitive parsers

Change API for creating custom parsers from: `Parsimmon.custom(stream, i, success, failure)` to `Parsimmon.custom(success, failure)`, letting the user return the stream parsing function by themselves.
Merge pull request #31 from khepin/feature/custom_parsers

Allow custom primitive parsers