Add more startup flags / URL parameters

Use case: In https://www.reddit.com/r/Seattle/comments/nlprut/biking_hills_map/, it would've been nice to toss &layer=steep_streets in the URL.

Any other shortcuts missing and likely useful? Maybe info panels too -- &info=b47, with the value using https://github.com/a-b-street/abstreet/blob/9fc79b1089dbe8ff8a881325f9e3a4f4c30c1e8f/game/src/common/warp.rs#L165

Code pointers:

• URLManager::update_url_free_param
• Flag parsing: https://github.com/a-b-street/abstreet/blob/9fc79b1089dbe8ff8a881325f9e3a4f4c30c1e8f/game/src/lib.rs#L62