Allow toolbar to be multiline

[noirbizarre]

Sometimes, depending on the number of enabled buttons,
it's necessary to split them on two (or more) lines.

This PR allow to specify multiline toolbar buttons by nesting arrays:

var editor = new MediumEditor('.editable', {
    toolbar: {
        buttons: [
            ['bold', 'italic', 'underline', 'strikethrough', 'superscript', 'subscript', 'pre'],
            ['h2', 'h3', 'justifyLeft', 'justifyCenter', 'justifyRight', 'justifyFull', 'quote'],
            ['orderedlist', 'unorderedlist', 'outdent', 'indent', 'anchor', 'image', 'removeFormat']
        ],
    },
   buttonLabels: 'fontawesome'
});

A demo has be added to see the rendering for each theme.

Some examples with flat, bootstrap and tim themes:

[coveralls]

Changes Unknown when pulling 14bdf87 on noirbizarre:multiline-toolbar into * on yabwe:master*.

[zuck]

Any chance to get it merged?