MediaWiki:Common.js

var customizeToolbar = function() {
$('#wpTextbox1').wikiEditor('addToToolbar', {
        section: 'advanced',
        group: 'format',
        tools: {
                "strikethrough": {
                        label: 'Strike',
                        type: 'button',
                        icon: 'http://upload.wikimedia.org/wikipedia/commons/3/30/Btn_toolbar_rayer.png',
                        action: {
                                type: 'encapsulate',
                                options: {
                                        pre: "<s>",
                                        post: "</s>"
                                }
                        }
                }
        }
});
alert('done');
};

if ( $.inArray( mw.config.get( 'wgAction' ), ['edit', 'submit'] ) !== -1 ) {
  $(document).load( customizeToolbar() );
}