fontSize parameter should be working for single node

[volkantas]

Is your feature request related to a problem? Please describe.
Yes, node.fontSize parameter is not working for single node.

Describe the problem
Go to demo page
Add new node parameters as 'fontSize: 20'
You will see nothing will be changed on demo app

Describe your use case
node.fontSize should change font size of node.

[danielcaldas]

Here is the source where we read the fontSize from the config object. We also want individuals nodes to be able to specify this value, therefore they should have precedence over the global value (just like we do for many other properties, e.g. node.svg)

This issue has a very narrow scope and it's a very good opportunity for new people to contribute to the project (of course people with experience please do feel free to take the issue as well 😉 ).