Added a convenience method

I added a method that takes two Vector3 objects, unpacks them, then calls the "real" line method
line convenience method

- added a Vector2 line convenience method
- added a 3D line method that takes colors
- all `line()` methods are now final (since they're "just" wrappers) and now feed into 3D w/Color method
removed assertions from `line()` methods
Merge pull request #496 from g-pechorin/patch-1

Added a convenience method