Better room modification CLI arguments; allow --description modification

Room modification arguments were sort of a shambles: there were various
"action" items like "--set-perms" or "--add-moderator", but those
options were exclusive (meaning you couldn't, say, add a moderator and
delete another moderator in the same command).

This overhauls how modifications work, accomplishing the following:
- removed the `--set-perms` argument; it is implied by having any of the
  `--add-perms`/`--remove-perms`/`--clear-perms` options.
- `--description` can now be used to update a room description (when
  used with `--rooms` to specify the room).  Previously it was only used
  when adding a room.
- Modifying options (like `--description`, `--add-moderators`, etc.) are
  no longer mutually exlusive: one command can now add/delete
  moderators, set permissions, and change the description all in one go.
- Simplified various identical logic around how `--rooms` is handled.
- Add '\n' and '\\' escapes to the `--description` argument so that you
  can add newlines into the description without having to fumble around
  with passing literal newlines in cli arguments.

Author: jagerman