Add support for showing source code when linking to file in a GitHub project

[jhthorsen]

Examples:

• convos-chat/convos@8c640f8
• convos-chat/convos@8c640f8#diff-54e3d6f073701fdb5e85974c896dbe3cR9
• convos-chat/convos@da0505e

[jhthorsen]

Will not render diff, since I don't think that will be a good user experience. Did however add support for rendering single files.