Add list set (union, difference) commands

[JesseWeinstein]

This is useful for creating a pair of lists, one of which contains accounts you want to only look at irregularly, and the other containing everyone else you follow. You can do so by first doing a union of all your followers (using the "[following]" magic name) then doing a difference with the list of accounts to exclude.

I'm very open to suggestions about a better way to achieve this, but wanted to make it available in case it is of interest.

[craigmaloney]

I've given this some thought. I'm not 100% convinced of the utility of the list commands in here. Would it be possible to split off the configuration and follow changes into a separate pull request so we can discuss these separately? It's not that I'm opposed to the new commands, I'm just not sure they're something to add quite yet.

Thanks!

[JesseWeinstein]

Sure, I'll make a separate PR with the fetch_remaining (which probably should be applied to a few other places, too) changes and I'll toss in the get_mastodon change. Should probably be able to make it today.

[JesseWeinstein]

Regarding the new list commands, while I think the use I'm making of them (to make a all-followers-except-a-list list) is good, I'm also not sure if this is the right way to expose that functionality. Glad for any further thoughts you have about them.

[JesseWeinstein]

Actually, I'm just going to rebase my other cleanup PR, #222, back on develop, and merge in the non-new-commands changes into that.