Add 'is_nonconnected_pac' flag/filter or other indication of filer type to /committees/

[lbeaufort]

To filter non-connected committees for the nonconnected PAC spending visualization chart, we should enable exclude capabilities for /committees/ org_type filter. Also, if it's a more elegant solution, we can consider modularizing the "add exclude" process.

Info on nonconnected committee type, org type, and designation: https://docs.google.com/spreadsheets/d/1XfOcgSXiMAKZxewwjj6SUZGo0UplKOzQUqFyCi-bF2w/edit#gid=0

Completion criteria:

• Consider using TDD with this example and adding a test first
• Enable exclude capabilities for /committees/ org_type filter (the validation list doesn't include '-C', '-L', etc).
• Consider modularizing this to make it easier to add exclude ability to other fields.
• Make sure the endpoint can perform adequately with excludes like this one: https://api.open.fec.gov/v1/committees/?api_key=DEMO_KEY&sort_hide_null=false&sort_nulls_last=true&committee_type=O&committee_type=W&committee_type=V&committee_type=U&committee_type=Q&committee_type=N&cycle=2018&cycle=2020&organization_type=-C&organization_type=-L&organization_type=-M&organization_type=-T&organization_type=-V&organization_type=-W&sort=-committee_type_full&per_page=30&page=1
• Add a test