Add support for generating shell completions

**Describe the feature you'd like**

It's pretty common these days for programs to bundle their own shell completion generators. It would be great if `cloudflared completion $SHELL` emitted the completion function for the specified shell.

**Describe alternatives you've considered**

Could write a shell completion function by hand, but there's a lot of overhead to do that and then to distribute it so that people can use it.

**Additional context**

IIUC, urfave supports bash (and maybe fish) completions. That probably covers a lot of cases.