add support for setting custom modifiers in the options

commands can now be functions as well as strings, like so:

	new Medium({
            element: document.querySelector('article.content section.editable'),
            autofocus: true,
            placeholder: 'Write some text.'
            modifiers: {
                188: function() { /* do something here */ }
            }
        })

(cherry picked from commit 2f38e361fe1d7f01845097909b0b5cc1e2fa3464)