Set parser at parse time

[tfwright]

Currently it seems that it is only possible to specify a parser for MultiXml to use at the class level like so: MultiXml.parser = :ox. However, this makes it quite awkward to use different parsers in the same application. It would be preferable to allow the parser to be set for individual parse operations like so: MultiXml.parse(xml, parser: :ox). If there's support for this idea, I would be happy to put together a PR.

[sferik]

Hey! If you still want this feature I’d accept a pull request that supports it. Sorry for the multi-year delay.