Add "fork me on GitHub" banner component

[cjbrooks12]

Many project docs websites want to show the "Fork me on github" type banner. An Orchid component should be able to add that banner to any theme. It should be added to the OrchidGithub plugin.

Features:

• Support both classic and modern styles (use modern by default)
• Component configuration allows banner to be placed in all 4 screen corners
• Support both fixed and absolute positioning
• Change color of banner and text (use colors from theme by default)