Add 'active_format' parameter to bp_get_member_last_active()

This new param allows theme authors to specify whether they'd like the current
user's last activity string in the format "Active 5 minutes ago" vs the simpler
"5 minutes ago".

Fixes #5387

Props lenasterg

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7846 cdf35c40-ae34-48e0-9cc9-0c9da1808c22