Removed `--remove` and `--no-quit`; replaced w/ inference. Solves #181.

This removes the two command line options `--remove` and `--no-quit` and. Instead, inference is used:

* If an output file is specified, WebTorrent won't remove the files after termination/after torrent is done.

* WebTorrent will now **keep running** if it's streaming (Chromecast, AirPlay, HTTP server, etc.) but **only** if it sees at least one request to the server coming. 

 * *In other words, if an HTTP server is instantiated but it never gets any requests, WebTorrent will choose to terminate after the download is done.*