MediaWiki:Common.js

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

addOnloadHook(customizeToolbar());