Create filter by post/term meta in dashboard pages

[leonofre]

Create filter by post/term meta in search pages.

Main feature: Create a hook to add filters by meta fields on the search query.

Description: The list is generated in display() method on the WP_Terms_List_Tables class that extends the WP_List_Table class, this method calls display_tablenav() method where bulk_actions(), pagination() and extra_tablenav() methods is called. This must work like extra_tablenav() method on WP_Posts_List_Table class.