Add filter to modify Liveblog output location

Fixes: #525

Some users would prefer having Liveblog output at the top of their posts rather than the bottom. This filter introduces the simple functionality to choose whether the Liveblog output should be above the content or below it (default).

liveblog.php

@@ -1169,6 +1169,10 @@ public static function add_liveblog_to_content( $content ) {
 			$liveblog_output = '<div id="wpcom-liveblog-container" class="' . self::$post_id . '"></div>';
 
 			$liveblog_output = apply_filters( 'liveblog_add_to_content', $liveblog_output, $content, self::$post_id );
+
+			if ( false === apply_filters( 'liveblog_output_at_bottom', true ) ) {
+				return wp_kses_post( $liveblog_output ) . $content;
+			}
 
 			return $content . wp_kses_post( $liveblog_output );
 		}