Adding preserve_border argument to simplify_mesh_lossless method

[Kramer84]

The preserve_border option is an important feature when dealing with non watertight meshes, or when all the points on the open edge are co-planar for example, and where modifying them would create later issues.

This options is not implemented for the simplify_mesh_lossless method, but it should be possible to add it, given the simple modification presented in this issue , and that is already applied for the usual simplify_mesh method.

[Kramer84]

Is resolved in PR 37