Callback beforeOpen to execute code before showing menu

[mc7244]

This adds a callback which can be provided with:

$("#whatever").contextPopup({
beforeOpen : function(e) {
// Content
},
items: [
// ...
]
});

This is very useful in situation where you want to change something before actually showing the menu. For example I use it to select a table row (i.e. paint it orange) before showing the menu for it.

[joewalnes]

Hi Michele

Thanks for the contribution. Please could you add an explanation of the new feature to the comments in the file header, and README.

Thanks
-Joe

[mc7244]

Ok! I opened another pull request with the patch including documentation.