Ability to click submenus

[zyuhel]

occasionally It is a useful ability to click on items containing submenus. For example if menu is used for tree categories selector.
I have made a Pull request #481 adding option for this.

[bbrala]

Thanks! Merged the branch.

[CipherTrustee]

@bbrala

There may be a regression on this feature...it fails because the key for an item that is a submenu is unknown at the time of a click event:

// abort if the key is unknown or disabled or is a menu

 if (!opt.items[key] || $this.is('.' + root.classNames.disabled + ', .context-menu-separator, .' + root.classNames.notSelectable) || ($this.is('.context-menu-submenu') && root.selectableSubMenu === false )) {
                    return;
}