Introduce 'latest_format' param for bp_get_blog_latest_post()

This param allows themers to omit BP's 'Latest post: %s' wrapper around the
link to the latest post.

Fixes #5392

Props lenasterg

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8408 cdf35c40-ae34-48e0-9cc9-0c9da1808c22