added superscript and subscript commands to wysihtml

We want to adopt wysihtml at work. One of the features we wanted was subscript and superscript support. I tested and compiled this locally. To create a subscript or superscript control, add these lines to your toolbar:

````html
    <a data-wysihtml5-command="superscript" title="sup">superscript</a>
    <a data-wysihtml5-command="subscript" title="sub">subscript</a>
````

Please let me know if I missed anything in regards to testing or project conventions. One thing I found when deploying locally is that I needed to also reference the toolbar.min.js lib like this:

````html
<script src="wysihtml/parser_rules/advanced.js"></script>
<script src="wysihtml/dist/wysihtml.min.js"></script>
<script src="wysihtml/dist/wysihtml-toolbar.min.js"></script>
````

it appears thats the docs only specify the parser and main wysihtml lib.