Make `$slug` and `$post_id` option for delete/format/import/load/update methods

[frozzare]

Methods:

• import_value
• delete_value
• format_value
• load_value
• update_value

Before:

public function update_value( $value, $slug, $post_id )

After:

public function update_value( $value, $slug = null, $post_id = null )