Super Simple ANET A6 E3D Carriage with 12mm Sensor Holder

Added a 12mm sensor for [Super Simple ANET A6 E3D Carriage](https://www.thingiverse.com/thing:2448062).

I've also added more mounting holes for the x-end stop.

ToDo:
* 18mm Sensor Mount
* BL Touch Sensor? Maybe.
* Clearance Check

Note: I haven't test printed this yet, still need to check some clearances when using the stock X-Axis