Support issue attachments

Can use the same mechanism as comments, ideally with common elements refactored to utility routines.

Implemented by @vyrondrosos as part of GSoC 2019.

Hi. I'm a little confused. This issue says that "issue attachments" was implemented by @vyrondrosos as part of GSoC 2019, but looking at the source code, it appears that it is not implemented yet:

git-issue/git-issue.sh

Lines 1473 to 1476 in a281489

    
           attach) # Attach (or remove with -r) a file to an issue. 
        
             echo 'Not implemented yet' 1>&2 
        
             exit 1 
        
             ;;

Is this feature implemented?

Hi @vyrondrosos, do you remember the story here?

	attach) # Attach (or remove with -r) a file to an issue.
	echo 'Not implemented yet' 1>&2
	exit 1
	;;