export withlevel

[oxinabox]

I feel like since the Logging std lib exports with_logger
we should export withlevel, possibly with the snakecase with_level to match
(and deprecate the old name)

[oxinabox]

@quinnj thoughts, since you introduced it.
Why wasn't it exported initially?

[quinnj]

I can't remember exactly other than just trying to be conservative at first to make sure it panned out and worked as expected. I'd be fine exporting and renaming.

[quinnj]

done in #94