Fixes #171.

Fixes permissions error by using network_admin_url instead of
admin_url. Reverted back to admin_url since after testing it works fine
for MultiSite.
Added textdomain in `_n_noop` function
Merge pull request #173 from KingYes/master

Added textdomain in `_n_noop` function
Fix: spelling
Fixes #174.
Fixes #180. Fixes #182. Fixes #183.
Merge pull request #179 from tivnet/master

Fix: spelling
Fixes #178.
Fixes #176.
Fixes #185.
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
Allow adding a plugin parameter named `is_callable`

Here's a use case: Yoast's WordPress SEO Plugin and the Premium version. Mostly same plugin but different slugs. This allows you to say "no, the slug doesn't exist, but this function (or method) is callable, so the plugin must be active".

Here's the plugin array item passed to `tgmpa()`: 

``
array(
    'name'      => 'WordPress SEO by Yoast',
    'slug'      => 'wordpress-seo',
    'required'  => false,
    'is_callable' => 'wpseo_init',
)
``

If the `wpseo_init()` function exists, the plugin's still considered active.
Update class-tgm-plugin-activation.php
Merge pull request #205 from zackkatz/patch-1

Allow adding a plugin parameter named `is_callable`
Fix WPCS formatting

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

Add filters to enable more customization