GitHub urls should be listed beneath the github field

New structure for the github url, we list them beneath the `github` field
so that we can loop through them. We will likely make authors work the same
way.
Adds support for mutliple GitHub repos to the project template

Every project page takes the first GitHub repo listed and makes it the
primary one, all others are listed below. This needs severe design
attention not included in this commit.
fixup! Adds support for mutliple GitHub repos to the project template
Merge pull request #67 from 18F/support-multiple-repos

GitHub urls should be listed beneath the github field