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

[jhthorsen]

Examples:

• https://github.com/Nordaaker/convos/commit/8c640f8d55ef7841e95765652eae71b086a43bbe
• https://github.com/Nordaaker/convos/commit/8c640f8d55ef7841e95765652eae71b086a43bbe#diff-54e3d6f073701fdb5e85974c896dbe3cR9
• https://github.com/Nordaaker/convos/commit/da0505e3e96bf5592dacdd7c4236b60560831862

[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.