New syntax to CLI "play" command and "watch_continuously" config

[imsamuka]

Most of what is implemented here, was discussed before on #484, which will be closed if accepted. If there's any disagreement here, please read the discussion there before.

On another note, my changes Engine.play_episode() don't break the episode_missing hook API: instead of passing a copy of the ep_iter iterator, only the first episode number is passed. But this severely limits the power of the API in the future. If permitted, i will break the API in favor of more flexibility with it (of course, updating the hook files in the repo as well).

[imsamuka]

Since no more input was given, I did some changes to conform to the avoidance of breaking changes.

• Absolute episodes are no longer prepended by #.
• Relative episode numbers are now prepended by n, standing for next.
• The empty "" range behavior is now dependent on watch_continuously.
• watch_continuously is now False by default, maintaining the original behavior.
• Improved the help message for play (debatable, but i think it's pretty intelligible)