Support for show all subcommand

[jtara1]

I want to be able to show all descriptions and all comments of all issues. It'd be nice to have more cli options for the subcommand, show, but I really just want to be able to dump all info from the issues section to a text file of some type. That'd be nice if it was saved cleanly in markdown file(s).

Some ideas for the possible command:

ghi show *
ghi show all
ghi show --all

It looks like this subcommand requires an issue number atm.

Also, show all for closed, open, or both types of issues.