Support multiple maintainers, default titles in updater tool

[thomashoneyman]

This PR addresses a couple irritations with the updater tool, namely:

• The default value of display-title is now the title-cased package name, so this flag no longer needs to be specified for most packages (there are occasional exceptions, like "FreeT", which will incorrectly render "Freet" by default, and which you'll still have to manually specify the --display-title flag)
• You can now specify multiple GitHub usernames as maintainers (ie. --maintainer garyb --maintainer thomashoneyman) instead of manually fixing the underlying README file.

It also moves @milesfrain's alias command into the Nix shell so that the whole process runs with just nix-shell and you can get right to using contrib-updater. I verified this works for me with a fresh clone, but I'd appreciate someone else confirming they can clone this repo and run nix-shell and start using the tool just fine.

[thomashoneyman]

I also noticed when updating the form-urlencoded library that there were some places we're using http instead of https in the templates. I've also fixed that.

[JordanMartinez]

Looks good to me!