Blender Git Loki

Solidify modifier: add option to assign shell & rim geometry to selected vertex groups.

This commit gives the solidify modifier the ability to assign the newly created shell
and rim geometries to selected vertex groups. This expands the procedural control over
the modifier stack by letting users apply modifiers to the shell geometry without affecting
the original geometry.

This will be especially helpful for NPR users that use solidify to create backface
culling lines on their characters giving them the ability to add displace noise
and other effects.

Differential Revision: https://developer.blender.org/D6903