The title can be shown either inside the hole (if there is one),
or outside. If the title is placed outside, the algorithm tries to
position it so that it does not intersect labels or pulled slices.

This commit changes titleposition to be of the form 'top left',
'top center', ... I added unit tests for correct placement in all
possible positions. I also added mocks using multiline titles.

If it is undefined, titleposition is set to 'middle center', if hole > 0,
and to 'top center' otherwise. _verifyTitle now returns a promise.