adds code injection

closes #1993

includes code injection for `{{ghost_head}}` and `{{ghost_foot}}`

Some steps still have to be done
* Include CodeMirror for syntax highlighting
* renaming settings "head" and "foot" to "ghost_head" and "ghost_foot" (to be on par with the handlebar helpers)
* and removing all the comments resulting from the copypasta :-)
* related: TryGhost/Ghost-UI#86

@DominikAngerer will look into CodeMirror.