Allow nodes to override strokeColor

[danielcaldas]

Right now some of the config properties have node granularity which means that each node can within
itself override the global config.

Clients should be able to override config.node.strokeColor at node level.

For that, we should allow the node builder to get these properties from the node first instead of fetching them
directly from the global config. So basically as we do in config.node.color:

let fill = node.color || config.node.color;

We should have the same for config.node.strokeColor.

Please remember that:

• configs are documented on top of the graph.config.js file
• Updating the config to be node granular means that we need to add the symbol 🔍🔍🔍 to that property in the
  config documentation

A good place to start with this issue would be graph.helper.js since is there that node and link properties
are calculated!

Happy Hacktoberfest!

[Andras-Simon]

@danielcaldas Can I take this?

[danielcaldas]

Godspeed you @Andras-Simon! Go ahead!

[danielcaldas]

Closing with #123