Added `weaver completion` bash completion command.

[mwhittaker]

This PR introduces a weaver completion command that spits out a bash completion script that enables tab completion for the weaver command. For example:

$ weaver <tab><tab>
generate   gke        gke-local  multi      single     ssh        version
$ weaver single <tab><tab>
dashboard  deploy     help       metrics    profile    purge      status     version
$ weaver single p<tab><tab>
profile  purge

To enable the tab completion, you run source <(weaver completion). This mirrors kubectl completion bash. Source the file in your ~/.bashrc if you always want it to be enabled.

I added weaver completion selfishly, as I thought it was useful. If others find it useful, we can advertise it to users. There is a bit of overhead in updating it whenever commands change, and the gke commands could be stale, so it would require work to be more robust. Also happy to leave this out of the repo and have it privately for myself.