Add ability to test 'current' pager items in PL

[willthemoor]

What:

Adds a single new yml key to pager.yml: current: 2.

Why:

Allows a developer to visually test for the "current page" within the pager element.

How:

No other changes!

To Test:

• Set current to a positive value
• add a style for .pager__item.is-active and .pager__link.is-active
• look at it. :)

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