Make post metadata display pluggable

[beastaugh]

Currently it's quite hard to customise the display of post metadata, outside the limited options available for individual elements (dates, authors). Emmanuel Gravel has proposed a single action hook, th_metadata. My approach would more likely be along the lines of the tarski_navbar hook, where elements are added and removed via filters.

[beastaugh]

Add post metadata via the th_post_metadata filter hook.

This changes the signature of a significant number of functions and should be considered a major change.

Closed by 913891d.