Add `chain` command line option to sauron

[Dan Cline]

sauron currently has no easy way to listen for messages from a single chain, it should include an option like the following:

sauron --chain bsc

Mainnet should be the default parameter, but it might also be useful for users to input a chain I'd rather than a named chain:

sauron --chain 1337

Additionally, there should be a prompt that lets the user know that the relay is listening on ONLY that chain (or chains) if specified.

This could eventually be configured statically, with some sort of ranger.toml file.