Add a configuration file

[jwodder]

demagnetize should be configurable via a TOML configuration file containing the following settings:

• local port or port range to report to trackers
• whether to actually listen on (and reject incoming connections to) the local port
• the numwant value to send to trackers
• local bind address
• whether to use IPv4
• whether to use IPv6
• maximum number of magnet links to process at once in batch mode
• maximum number of trackers to connect to at once per magnet link
• maximum number of peers to connect to at once per magnet link
• tracker timeouts
• general peer interaction timeout?
• maximum size of incoming packets to accept from peers?

The CLI should also have a dump-config subcommand for outputting the current configuration with explanatory comments above each field (good for creating a new config file).