Accept vanilla Linux Foundation signoffs

This allows signing off commits with `git commit -s`

Should this; `github: ([a-zA-Z0-9][a-zA-Z0-9-]+)` be `github: ([a-zA-Z0-9][a-zA-Z0-9\-]+)` (escape the last dash) or `github: ([a-zA-Z0-9][a-zA-Z0-9\-]+[a-zA-Z0-9])` (don't allow trailing dash) – at least; I expect that is to allow dashes in GitHub names?

Note: I tested on [this site](http://www.regexplanet.com/advanced/golang/index.html), I didn't have access to a Go environment :)