Add autocompletion support

[fvictorio]

This is easy to to do, but I think we should wait until oclif/plugin-autocomplete#13 is done, otherwise the experience with eth repl abi@address is significantly worse IMO. That being said, adding autocompletion is completely optional, so if someone wants to start working on this, be my guest.

• Add autocomplete plugin
• Add instructions on how to use it in the README