Add tile padding as an option

[petebankhead]

This adds 'Tile padding' as an additional option.
This relates to #66

Ideally, we'd like to compute this automatically and avoid requesting it from the user. But we're still improving the tiling code, and at this point it's useful to be able to change the padding.

As a rule:

• Lower padding values are faster (especially when the tile size is also low)
• Higher padding values are necessary if you see objects being split/fragmented/otherwise weird on tile boundaries

In an image where all the nuclei are small (e.g. CMU-1.svs), a padding of 16 might be fine. But on an image with some huge nuclei (e.g. OS-2.ndpi), a larger value (e.g. 80) might be needed to avoid missing important cells.