Support for emitting multiple knit_meta objects and selective retrieval

Two small changes enhancing `knit_meta`:

- It is possible for a function to return more than one piece of metadata (for instance an `html_dependency` and an `rmd_warning`), so in the state accumulator we now just concatenate the lists

- It is possible to selectively access and clean the state in `knit_meta` we're interested in

These changes will support a feature in the R Markdown package that allows warnings to be collected during render.