Context menu support

[saskaale]

Write example for support context menu.

Basic iIde how to make is style div element to look like browser context menu.
Once user would right-click on the graph there would be call for the find() method. for nearby area (let's say 10px) If the area contains the node, look if the node has property for context menu, if yes then show the context menu with specified context.

At the beginning I'd start with the raw html ( string / element ) as parameter to the context menu context.

e.g.:

nodes = [{
label: "node_1
},
{
label: "node_with_context,
contextMenu: HTML_STRING_or_HTML_ELEMENT
},
]