Added data to wp_pagenavi filter to allow for custom html.

The current structure of the data array that is passed to the filter is:
```php
<?php
array (
  'current' => '1',
  0 => 
  array (
    'url' => 'http://example.com/page/2/',
    'text' => '2',
  ),
  1 => 
  array (
    'url' => 'http://example.com/page/3/',
    'text' => '3',
  ),
  2 => '...',
  3 => 
  array (
    'url' => 'http://example.com/page/6/',
    'text' => '6',
  ),
  4 => 
  array (
    'url' => 'http://example.com/page/9/',
    'text' => '9',
  ),
  5 => 
  array (
    'url' => 'http://example.com/page/12/',
    'text' => '12',
  ),
  6 => '...',
  'next' => 
  array (
    'url' => 'http://example.com/page/2/',
    'text' => '»',
  ),
  'last' => 
  array (
    'url' => 'http://example.com/page/14/',
    'text' => 'Last »',
  ),
)
```

So, by filtering the html like so:

```php
<?php
add_filter( 'wp_pagenavi', 'my_pagination', 10, 2 );
function my_pagination( $html, $pagination_data ) {

	$out = '';
	foreach ( $pagination_data as $key => $value ) {
		/** Set the default class. */
		$class = 'item';

		/** If this is a special pagination item, add the proper class. */
		if ( is_string( $key ) )
			$class .= " is-$key";
		/** If this is ..., add a special class. */
		else if ( ! is_array( $value ) )
			$class .= ' is-disabled';

		/** If this is a link, wrap in an anchor. */
		if ( is_array( $value ) )
			$value = "<a href=\"{$value['url']}\">{$value['text']}</a>";

		/** Wrap everything in a list item. */
		$out .= "<li class=\"$class\">$value</li>";
	}

	/** Return the list. */
	return "\n<ul class=\"pagination\">$out</ul>";
}
```

we can can output the pagination in a list:

```html
<ul class="pagination">
	<li class="item is-current">1</li>
	<li class="item"><a href="http://example.com/page/2/">2</a></li>
	<li class="item"><a href="http://example.com/page/3/">3</a></li>	
	<li class="item is-disabled">...</li>
	<li class="item"><a href="http://example.com/page/6/">6</a></li>
	<li class="item"><a href="http://example.com/page/9/">9</a></li>
	<li class="item"><a href="http://example.com/page/12/">12</a></li>
	<li class="item is-disabled">...</li>
	<li class="item is-next"><a href="http://example.com/page/2/">»</a></li>
	<li class="item is-last"><a href="http://example.com/page/14/">Last »</a></li>
</ul>
```

As you can see, __currently there is no distinction between "Previous …" items and "Next …"__ items. I can add this if you think it would be useful.

If a more formal structure is needed for the data array, the implementation can be changed to generate something like this:

```php
<?php
array (
  0 => array (
	'text' => '1',
	'meta' => array( 'type' => 'current' ),
  ),
  1 => 
  array (
    'url' => 'http://example.com/page/2/',
    'text' => '2',
  ),
  2 => 
  array (
    'url' => 'http://example.com/page/3/',
    'text' => '3',
  ),
  3 => array(
	'text' => '…',
	'meta' => array( 'type' => 'next_ellipsis' ),
  )
  3 => 
  array (
    'url' => 'http://example.com/page/6/',
    'text' => '6',
  ),
  4 => 
  array (
    'url' => 'http://example.com/page/9/',
    'text' => '9',
  ),
  5 => 
  array (
    'url' => 'http://example.com/page/12/',
    'text' => '12',
  ),
  3 => array(
	'text' => '…',
	'meta' => array( 'type' => 'next_ellipsis' ),
  )
  'next' => 
  array (
    'url' => 'http://example.com/page/2/',
    'text' => '»',
    'meta' => array( 'type' => 'next' ),
  ),
  'last' => 
  array (
    'url' => 'http://example.com/page/14/',
    'text' => 'Last »',
    'meta' => array( 'type' => 'last' ),
  ),
)
```

This version works well if other information about the items needs to be added in the future (just add it in the meta array).