Add Post ID to msm_sitmap_skip_posts

[BrookeDot]

This PR adds $post_id to the msm_sitmap_skip_posts filter. Doing so allows for posts to be excluded by using the filter directly.

Note: this is a backward incompatible change, as it adds a second argument to the filter

Fixes #183

[pkevan]

This will probably need to be tested with someone using the filter currently, as I can't find any examples!

[BrookeDot]

Makes sense @pkevan the other thought I had was to support both (adding a new filter for the addition of $post_id) something like msm_sitmap_skip_posts_by_id and then fire a doing_it_wrong when the old filter is used.

Thoughts?

[pkevan]

Probably preferable just to have one filter - in theory it shouldn't break by adding a parameter (as 1 parameter is the default for all filters), but depends on how legacy application have been calling it.