Execute this code in the console to add the `Cmd-I` shortcut for markdown italic to all CodeMirror cells in the page. Tested on Google Colaboratory.

CodeMirror-add-italic-shortcut.js

var script = document.createElement('script');
script.src = "https://ajax.googleapis.com/ajax/libs/jquery/2.2.0/jquery.min.js";
document.getElementsByTagName('head')[0].appendChild(script);
map = {'Cmd-I': function(cm) {
        var s = cm.getSelection(),
            t = s.slice(0, 1) === '*' && s.slice(-1) === '*';
        cm.replaceSelection(t ? s.slice(1, -1) : '*' + s + '*', 'around');
    }};
$('.CodeMirror').each(function() {
    $(this)[0].CodeMirror.addKeyMap(map);
});