Add filters to enable more customization

* Added `tgmpa_submenu_parent_slug` filter - modify what menu the sub-page is under, instead of requiring it under "Appearance". This allows the page to be added to a plugin's sub-menu.
* Added `tgmpa_table_columns` filter - modify table columns in table view
* Added `tgmpa_table_data_item` filter - modify plugin data that's ready for the table output
* Added `tgmpa_notice_rendered_action_links` filter - modify final action link output

I wanted to add a "Description" column that says why should an user install this plugin.

```
array(
    'name'      => 'WordPress SEO by Yoast',
    'slug'      => 'wordpress-seo',
    'description' => 'This is a valuable SEO plugin.',
);
```

These filters allow much more flexibility in the setup.