Add support for downloading upper and lower legislative districts

[rdmurphy]

👋

This adds support for downloading upper and lower state legislative districts as mentioned in #3.

I haven't actually smashed it against the doomsday device/download_everything call, but will give it a shot once I'm home! Quick tests of passing TX into AgeDownloader were successful.

[palewire]

Doomsday device is def. right. Pretty sure @ryanpitts thinks it's an abomination. An ad hoc CLI for downloading individual classes would be a good add separate from this.

[palewire]

Question. Does the Census API require you provide a state to this function, or could we download the entire country at once?

[rdmurphy]

@palewire Hah, see this was already merged, but I both checked 1) if download_everything worked (it did) and 2) you cannot download the entire country all at once for legislative districts, it expects a state.

[palewire]

Thanks. You're out with the new version on PyPI. I've also added a new CLI interface that's now in the README.