Allow plugins to add a custom column in Signup WP_List_Tables

Thanks to a new "column_default()" function in "BP_Members_List_Table" and in "BP_Members_MS_List_Table" (multisite), plugins will be able to add data rows to the eventual custom columns they created using the filters "bp_members_signup_columns" and "bp_members_ms_signup_columns".

Fixes #5678

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8472 cdf35c40-ae34-48e0-9cc9-0c9da1808c22