Add support for container padding

[stevenvachon]

• Padding is computed, which supports values with em and other units as they are automatically converted to px
• Top/Bottom or left/right padding is added to the sizer ("scroller")
• Top/Left padding (which precedes the first item) is added to each item's style position, but not the scroll position, thereby retaining the padding values when using scrollTo with _itemPositions. For example, scrolling to item 0 would have otherwise never reached the the beginning (0px).
• Some tests were updated because now the scroller has a top CSS value.

[tbranyen]

What was this change about?

[stevenvachon]

Previously, the sizer element (<tr>) didn't have a top css property, which met the test's condition for exclusion. Now that it does have that property, the condition needed to be updated.

[tbranyen]

<tr> can be changed via scrollerTagName so we should probably use that instead of hardcoding tr.

[tbranyen]

Very cool, looks great to me. I like how it only gets calculated during refresh. I'll review it in more depth and if anything sticks out will comment, otherwise we can merge.

[stevenvachon]

@tbranyen Any update? It'd be best if this didn't go stale.

[tbranyen]

@stevenvachon only had that one last comment about the tr and once that is resolved, I think we're good.

[stevenvachon]

@tbranyen scrollerTagName doesn't have a default value in the config, though; only in refresh().

[tbranyen]

@stevenvachon I think from a code perspective it looks good, but we need to add some documentation around the change in the README or even an example.

[stevenvachon]

I don't think this PR warrants a documentation change. It now simply behaves as you'd expect it to if the scrolling container were to have padding.

[tbranyen]

Published in 1.0.0