Added multiple value support to yajl2 backend

[davidfischer]

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 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.

[isagalaev]

Merged, thanks!

The approach is reasonable (although if there ever will be a third flag it'll make sense to generalize setting of config values with a loop). I don't think it makes sense to backport it to the yajl backend as it is maintained solely for backwards compatibility. And I'm -0 on adding it to the python backend. It's a rare feature and I'd rather add those only when and where they are needed. Same goes for other flags.

[davidfischer]

Sounds good.