Convert ellipsis string into HTML element.

Use an li element as the other pagination elements are wrapped in li by
default as well.
Remove separator element.

With modern CSS frameworks and CSS rules in general the separator
element is simply not worth its weight in most situations. The line
between pagination links can easily be handled through CSS borders now.

Not having this additional feature allows the internals to be simpler
with no loss of functionality in the majority of use cases.