Custom keyboard shortcuts

#### Default

> If Editor.md code editor is on focus, you can use keyboard shortcuts.
> Editor.md have the default keyboard shortcuts handle. Plaese open the help dialog, can see all default keyboard shortcuts.

#### Example

```javascript
var testEditor = editormd("test-editormd", {
    width: "90%",
    height: 720,
    path : '../lib/',
    disabledKeyMaps : [
        "Ctrl-B", "F11", "F10"  // disable some default keyboard shortcuts handle
    ],
    onload : function() {
        var keyMap = {
            "Ctrl-S": function(cm) {
                alert("Ctrl+S");
            },
            "Ctrl-A": function(cm) { // default Ctrl-A selectAll
                // custom
                alert("Ctrl+A");
                cm.execCommand("selectAll");
            }
        };

// setting signle key
        var keyMap2 = {
              "Ctrl-T": function(cm) {
                alert("Ctrl+T");
              }
        };

this.addKeyMap(keyMap);
        this.addKeyMap(keyMap2);
        this.removeKeyMap(keyMap2);  // remove signle key
    }
});
```