Support scroll padding

[richardscarrott]

Currently scroll padding isn't factored in when computing the pages.

Some common scenarios include:

Scenario 1:

.scroll {
  scroll-padding: 0 100px;
}

Scenario 2:

.scroll {
  scroll-padding: 0 100px;
  padding: 0 100px; /* Allows the scroll-padding to take effect on the first item */
}

Also need to handle scroll-margin which is declared on an item & I think is calculated in addition to scroll padding.

[richardscarrott]

Released in 0.3.0