allow django management commands to target an arbitrary core

[thatbudakguy]

Is your feature request related to a problem? Please describe.
Solr allows "swapping" cores, which is a convenient mechanism for enacting lengthy updates (e.g. reindexing) without any downtime. If solr_schema and index were able to target a different core, one could easily prepare the second core with the new data and then "swap" it with the production core to make the changes instantaneous.

Describe the solution you'd like
Both solr_schema and index accept a parameter that specifies the name of the core to operate on. Using -c might be a natural choice for parity with the bin/solr command, but -c is already in use to mean "clear" for the index command, so we'll need something else. Perhaps you would just need to write it out, like:

$ python manage.py solr_schema --core ppa