Adjust frame calculations to support negative minimumLineSpacing.

[J-Rojas]

Closes #30.

[keighl]

Hey @J-Rojas, appreciate the pull! Never seen negative line spacing in a collection view before. I imagine the overlap would be funny since the layer indexes aren't necessarily above previous rows, right?

Could try adding an example to the ./Example project that demos it? I'm interested in seeing how it works with variable-height cells.

[J-Rojas]

While you can use negative line spacing to overlap opaque layers visually, I'm using negative line spacing to overlap the insets within cells for a complex layout, so only the transparent areas are overlapping.

I'll add an example to your project when I can carve out more time, but hopefully my description above clears this up and why it's useful.