While the job fetching for bundles looks good, there's some stuff I'd like to have handled before declaring support for downloading bundles in the readme, etc:

• The client is able to fetch jobs, but not claim titles from the bundle. At least the bundles I own have this description set on the bundle page: Note: Projects in this bundle are hidden in your library by default until you first access them in order to avoid flooding your library. You can return to this page at any time to access any projects you wish to show in your library. - and sure enough, none of those games have their download keys available to actually fetch files. There should be a warning for this case, but I don't know what determines this behavior, of if it's all of them.
• What if someone provides a bundle URL with the download key? They look like itch.io/bundle/download/AAbca...1234 and don't contain the bundle ID in the link, but it could be scrapped by looking up the link on elements like <a href="/b/123/bundle-slug" class="return_link"> - this doesn't have to be supported right away, but a warning could be nice.
• What if the link w/ download key is claimed? Itch will throw a 404 if the page is requested while logged out. (Not sure if the API key authenticates web requests, probably not.)

See comments for the changes I'd like to see now, I'll merge it with those and add issues to deal with the remaining stuff later.

@jackwilsdon I'll have some time this weekend for itch-dl, would you like to finish this PR or can I take it as is and fix up what's left?

I don't think I'll have any free time to work on this PR this weekend sorry - feel free to take it 👍