Provide support for origin source URLs

[jroper]

Lagom (and several other projects) render links back to the original markdown source files in GitHub to facilitate user contributions, see the bottom of this page for an example:

http://www.lagomframework.com/documentation/1.2.x/java/WhatDoesLagomMean.html

Paradox needs to support this such that a theme can render that link.

[dwijnand]

I believe this to be incredibly useful and should definitely support it sooner rather than later.

[eed3si9n]

This was fixed in #69