Print model on surface and add supports

[arusso]

@Scamp I believe this request is addressed by the most recent Preform update! You can review the list of features on this post.

PreForm 2.18.0 - October 3, 2018

PreForm 2.18.0 is a recommended update, which includes several new features and several changes that focus on improving the efficiency and versatility of support structures. New: Print directly on the build platform. Enable “On Build Platform” in the Supports menu to print parts directly on the build platform and to generate supports only where necessary. These supports adhere to the build platform with small pads, instead of the typical raft (or base). Replace model. When a source STL or OBJ…