Add bw_post_array filter to enable plugins to alter the titles of posts being listed

[bobbingwide]

For officialcaravan.co.uk we need to display a list of posts which are Other versions of a track.
The user doesn't get much value add when the titles are all the same
So we need to be able to extend the logic to enhance the post titles.

For this we'll add a new filter called bw_post_array. So named since it's invoked by the function called bw_post_array().