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.
Removed hard-coded "description" setting
Updated admin_menu() method functionality

* Removed `tgmpa_submenu_parent_slug` filter - now filtering all args 
* added `tgmpa_admin_menu_args` filter
* added `tgmpa_admin_menu_use_add_theme_page` filter
Fix WPCS formatting

Also merge in the `is_callable` commit.
Merge pull request #188 from zackkatz/master

Add filters to enable more customization