Added config setting for grabbing sponsors

Existing set up does not grab sponsors for Philadelphia legislation because the sponsor list for Chicago and Philadelphia are formatted differently. For Chicago sponsors are listed as links to specific legislators, in Philadelphia the list is a comma-separated list of council members.

This pull request does a few things:
- Adds a setting in the configuration for whether or not the sponsor is a list of links. By default this is set as true (should preserve backwards compatibility then).
- LegislatorScraper now uses the `sponsor_links` setting to determine how to parse sponsors for legislation
- Adds a test for grabbing sponsors for Philadelphia legislation

Additionally, I went ahead and converted existing tests to use `Config` and `DEFAULT_CONFIG`. This solves the issue that as new city specific differences are found and new config settings are added existing tests should pass without having to add defaults for all the new options added.