Added multiple value support to yajl2 backend

Some APIs seem to use a stream-like approach where JSON objects are just concatenated onto one another. Yajl supports this with a [multiple_values](https://github.com/lloyd/yajl/blob/master/src/api/yajl_parse.h#L151) flag but this was not exposed by ijson so I added it.

There are a couple things to think about here:
* Should this be added to the other backends or should it only be for yajl2?
* Should the remaining flags also be exposed?
* Is an optional parameter for each flag the best approach to exposing these flags?

No decision needs to be made immediately on these. As for this pull request, I just stuck to the same pattern that I saw in the code for allowing comments.