Implement notification subcommands #389

enhance notification listing
- add --states and --type filters
- toggle per-user or per-repo listing via --mine flag
- print more fields
add subcommands to mark notifications as read, unread, pinned, unpinned. operates on
- all notifications matching the --state and --mine filter flags, or
- a notification specified by ID.
~~add a --fields flag for notifications listing.~~ not in this PR
~~interactive mode~~ not in this PR. this would go well together with #324

fixes #243, fixes #155

based on initial work in #283 and #386, but opening a new PR for @6543 to review as I changed quite a lot

tea notifications --all has moved to tea notifications --mine
tea notifications now only works with the context of a remote repo.
To run this outside of a local git dir, run either tea n --mine or tea n --repo <my/repo>

aside: while developing i discovered a couple of issues with the notifications APIs:

nice refactor and works great!

one thing: why did you rename --all to --mine ?

☝️ noerw: "... for consistency with `tea time --mine` ..."

I can live with that ;)

looks good