Add support for generating shell completions

[jpeach]

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.