asdftool edit subcommand

[eslavich]

I think it would be useful to implement an asdftool edit CLI command. In the vein of crontab -e, the command would copy the YAML portion of the file to a temporary location, open up that temp file in EDITOR, and overwrite the tree in the original file once the user has exited the editor.

This has some advantages over the explode/edit/implode method -- it's a single command, and if the new tree fits within the space allocated to the old tree, then the file can be updated in place without copying the binary blocks or invalidating the block index.

This has been mostly implemented by #873, all we're missing is the bit that automatically launches EDITOR.

[eslavich]

This has been completed by #922