Add content type change list item for webhook instructions on GH.

This used to be set to a compatible type of `application/vnd.github.v3+form` but the new default is `application/json` and does currently work with this type.
Merge pull request #92 from Martii/UpdatedWebHookInstructions

Add content type change list item for webhook instructions on GH.
Make sure the language value provided by Markdown fence syntax is a language supported by hightlight.js. Closes #94.