Use 'g:gissues_default_remote' if none available

[sanmiguel]

Howdy @jaxbot - long time!

When figuring out which remote to use during getRepoURI(), it seems to default to the first remote in the (alphabetically-sorted) git remote -v output... I noticed also that the plugin has a config item, g:gissues_default_remote, that doesn't seem to get used...

I've got a proposal for a patch which I'll push and PR momentarily. :)