Add support for specifying a timeline source for the digest

[BenWard]

As someone running a solo personal instance at benward.social, I wanted to use mastodon digest to discover posts from beyond my follow graph, therefore I've added a flag that allows switching to any timeline source supported by the Mastodon.py library:

Use -f (for 'feed') flag when running to switch the timeline source for the digest (defaults to 'home').

You can specify:

• -f local (local server timeline)
• -f federated (public/federated server timeline; useful for people on very small or personal instance servers)
• -f list:n (a list, where 'n' is the ID, e.g. -f list:2)
• -f hashtag:tag (a hashtag, where 'tag' is a hashtag on your server, e.g. -f hashtag:Introductions)

For my use case, I run with -f federated to get a digest of every posts that's passed through my server; but since I'm the only user, it's a quite interesting summary.

Other changes:

• Since some of these timelines can be lower density, changed the run behaviour to exit with error 1 and a message when no posts are returned from the query, rather than rendering an empty template. This has a side-effect that anyone running the script via cron will maintain their previous, stale digest until new posts meet the threshold.

[hodgesmr]

Thank you! This is a great feature add. I've merged in a couple (admittedly, opinionated) style adjustments, but this is now merged in 😄