Handling default attributes for node/edge/graph statements

[ress]

I saw this was a feature request for dig.js and implemented it for dagre.

Default attributes apply to all statements following them:
digraph G {
n0 [label="colorless node"]
node [color=green font=Helvetica];
n1 [label="green node"]; /* color=green and font"Helvetica are added automatically */
}

Multiple default attribute statements are merged, and newer attributes have
precedence over older attributes:
digraph G {
node [color=green font=Helvetica];
node [color=yellow];
n0 [label="yellow node"]; /* font=Helvetica is added automatically */
}

[ress]

Fixed a bug in my initial commit. Works well now and includes test cases.

[cpettitt]

Thanks for adding this feature! I've changed the behavior slightly for how we handle nodes. If this is the first time we encounter the node (explicit node statement or implicit via edge statement) we try to apply the default attributes. Otherwise we only attributes that were explicitly defined in the statement. I believe this is consistent with how graphviz handles attributes.

[ress]

That change makes sense, and also makes the code simpler. Thanks for merging!