Make it possible to add non-boundary constraints that don't affect hole detection

[artem-ogre]

Make it possible to add constraints that don't affect triangle removal and hole detection

See #119

This PR introduces a flag on constraint edges that allows the distinction between "outer" and "inner" constraint edges, that is, edges that will be boundaries when removing the super triangle and holes, and edges that will simply be guaranteed to be part of the triangulation, but do not describe the boundaries of the triangulation, and will be ignored during "hole-detection".
I did not find any other way to achieve this, but maybe there's a much simpler way?
In any case, I wanted to publish our changes, and share them in this way. If you want to merge this, I'll gladly work in any comments that you might have.