Add ability to do IRSA dust query on multiple objects at once

The [IRSA dust service](http://irsa.ipac.caltech.edu/applications/DUST/) has an "Upload Table" option which should be an easy way to paralellize IRSA dust queries. Doing that should satisfy #682 for IRSA dust.

I think this is probably best implemented as a new function/method, maybe `IrsaDust.get_extinctions` or similar, but it _might_ instead make sense to instead make it part of `IrsaDust.get_extinction_table` if a non-scalar coordinate is input. The output table would look quite different, though, so that makes me think a new method is better.

I might be able to do this in the next few weeks, but I'm not certain, so if any one else wants to volunteer, please say so!

Start by reading the existing IrsaDust.get_extinction_table behavior and the IRSA dust service's Upload Table option. Decide whether the multi-object query belongs in a new IrsaDust.get_extinctions method or in the existing method for non-scalar coordinates. Done means supporting one uploaded table for multiple objects and returning the proposed multi-object output while satisfying #682.

Written by the indexing model from the issue text.