Add ignoreForClass attribute that aborts contextmenu if applied

[felixruponen]

• If the element has the ignoreForClass the context menu won't be shown

This adds ability to dynamically disable contextMenu

Example:

If the div has "some-class" the context-menu won't show.

[josetaira]

I'm not sure using a class to disable this is the way to go. The more appropriate way to do so appears to just set a flag through an attribute like context-menu-enabled. Then this flagged is set by the controller based on some parameters, in this case the class.

@Templarian not sure if this is something we should merge in.