ability to filter with_front arg

[mrwweb]

This introduces a staffer_with_front filter to change the staff post type's rewrite['with_front'] arg. This is one possible way to resolve #4. With the filter, this is all that's needed to change the permalink structure:

add_filter( 'staffer_with_front', '__return_false' );

That's much better than the bulkier alternative given in #4.

That said, I wonder if a new option (defaulted to true) would make more sense. If you've never had complaints about this issue before, then a filter is probably sufficient, otherwise, an option is probably better and this pull request should be rejected.

Like I said at first, I think 'with_front' => false is probably a more sensible default but changing it at this point would probably break sites.

[edwardr]

I have some additional changes coming too. I still plan to review these this week. Looking to release the next update by end of the weekend.

[mrwweb]

Sounds great. Given what I wrote, consider this just an idea and take it or leave it depending on your strategy. The goal is just to ensure there's a way where the permalink structure just doesn't end up with things like /blog/staff/person/.