Add method for determining if viewing the current page

[justintadlock]

Currently, determining if we're viewing the current page is done through a combination of checking if we're viewing the last item and whether the last item should be shown. There are some scenarios where it's possible this is not technically the current page. This should be a rarity and not happen with the default plugin output, but it can happen with custom implementations of the crumbs array that's passed into the Markup classes.

I've known this was needed for a while, but this related ticket exposes the need for it further:

• Add aria-current label for accessibility #11

[justintadlock]

This could potentially be a method on the Crumb class. Something like $crumb->isCurrentPage().