Enhance comment footer on PR messages

[dpordomingo]

slightly related to #306 (same context)

Currently there is no an easy way to know where a lookout message come from, so different messages from different analyzers looks the same.

We could itterate a lot about it, like adding a picture of the analyzer, or different templates for them, but I think we're not there yet.

But we could, at least, identify the analyzer sending the message, using the footer on every comment posted by lookout.

[dpordomingo]

@se7entyse7en current footer comes from provider/github/poster.go#L184

[carlosms]

Maybe we can use a text template? I haven't tested anything, but I have in mind something like:

providers:
  github:
    comment_footer: "_If you have any feedback for this comment made by the analyzer {{.Name}}, please, [tell us]({{.Feedback}})

This could be a base to build comments similar to the ones proposed in #306.