Add support for non-lazy parsing

[ecton]

After considering the comments on #11, I've decided to try adding an option to offer non-lazy parsing. For JsonString, it'll require a new code path for string parsing.

I'm not really sure what to do with JsonNumber yet -- it's in an awkward state right now anyways. It probably needs to have the same flexibility as JsonString, by being able to be represented internally by a Rust type or by the JSON representation when in lazy mode.