Add options for setting default node and edge styles

[jlaw9]

Right now we set node and edge attributes for each node and edge separately. Most of the attributes we're setting are the same for all nodes and edges. It would be much cleaner (and probably faster) to set those attributes as defaults in a node or edge selector style attribute.

For example:

{
    "selector": "node",
    "style": {
        "shape": "ellipse",
        "background-color": "#666"
   }
},

[sandeepm96]

An add_style() method has been implemented to solve this issue. You can see the changes here:
077d470