Add filter for Webmentions title

Adds independent_publisher_webmentions_title filter.

Also removes independent_publisher_pingslist_* filters from README.md as
those were removed when webmention support was added in v1.7.

README.md

@@ -403,8 +403,7 @@ WordPress Filters and Actions allow you to modify the theme without actually mod
- `independent_publisher_entry_meta_author_prefix` - Allows you to override the default Author prefix (default is 'by'; e.g., "by <author name>").
- `independent_publisher_search_stats` - Allows you to override the Search stats shown on Search pages.
- `independent_publisher_tag_list_title` - Allows you to override the default Tag List Title of 'Related Content by Tag' at the bottom of Single posts.
- `independent_publisher_pingslist_title` - Allows you to override the default Pings List title of 'Readers who Shared This' at the bottom of Single posts.
- `independent_publisher_pingslist_end_note` - Allows you to override the default Pings List end note of 'Thank you!' at the bottom of Single posts.
- `independent_publisher_webmentions_title` - Allows you to override the default Webmentions title of 'Webmentions' at the bottom of Single posts.

### Action Hooks

comments.php

@@ -97,7 +97,7 @@

		<?php if ( independent_publisher_comment_count_mentions() ) { // If we have mentions, let's show them ?>
			<div id="webmentions" class="mentions-list">
				<h3><?php _e( 'Webmentions', 'independent-publisher' ); ?></h3>
				<h3><?php echo apply_filters( 'independent_publisher_webmentions_title', __( 'Webmentions', 'independent-publisher' ) ); ?></h3>
				<?php independent_publisher_mentions(); ?>
			</div>
		<?php } // end if ( independent_publisher_comment_count_mentions() )?>