Allow plugin listings without downloads

[hypeJunction]

Until better github integration is implemented, perhaps we should allow plugins to be listed with a publically accessible URL (e.g. a download link, repo link etc).

[beck24]

I think if you only give the repo link for example, you'll end up getting more comments/questions about it not working when someone installs it without getting vendors for anything that uses composer. The people who use plugins often aren't developers themselves.