Add "Edit on GitHub"

[rgaiacs]

Raw of proof concept screenshot

This is related with #97 and the discussion of how to increase contributions to the lessons.

In the past we used GitHub Ribbons but this was deprecated on the new style for one reason that I don't know.

cc @ErinBecker

[wking]

On Mon, Feb 06, 2017 at 09:30:03AM -0800, Raniere Silva wrote: In the past we used GitHub Ribbons, but this was deprecated on the new style for one reason that I don't know.

I think we should use the ribbons (they're a more familiar UI than rolling our own), but we probably want to make this fix in swcarpentry/styles so both lessons and workshops can share it. Workshop repos get less development, but putting the ribbon up there is still useful as a “look, we dogfood this stuff” example to learners.