Add constants to define position tags

Hello,

I've added new constants to define the position tag ('first', 'current', 'more', etc...). These constants are defined in the Pagination class (```TAG_FIRST```, ```TAG_CURRENT```, ```TAG_MORE```, etc...).

If you accept this merge request, can you release a new tag (like ```1.0.1```), to be used in Packagist? :smiley: