R1:f52bc073216e

Make the Remarkup Help Page Extensible

Summary:
The Remarkup Help page is currently pointing to a HTML page on we.phorge.it.
Because of this it can not be customized/extended by custom extensions.
This diff allows you to implement a `RemarkupSyntaxDocumentationProvider` interface
to your custom class, which will implement a getDocumentation method.
This getDocumentation should return the custom help text in Remarkup format.
This diff also contains some extra help for the cowsay and figlet interpreters.