I've started to implement the arraymancer backend into the actual code of ggplotnim now.
Most recipes are compiling and working fine now. The rules for formula creation have changed a little bit, but actually provide more control now. There's no documentation about the rules until everything works, except: [https://github.com/Vindaar/ggplotnim/blob/arraymancerBackend/playground/arraymancer_backend.nim#L946-L958](https://github.com/Vindaar/ggplotnim/blob/arraymancerBackend/playground/arraymancer_backend.nim#L946-L958) and the modified recipes. The code currently uses the arraymancer backend by default (-d:defaultBackend to use the old one; yeah irony).
