Added custom css class on edge labels

[kaelhem]

Hi,
I've just made a small update on edge label to be able to specify custom css classes.

---

This PR allow to add a labelClass parameter on the setEdge function.

// sample :
g.setEdge(link.source, link.target, {
  labelClass: 'cutomClass1 customClass2'
})

The original label class is untouched.

It should fix #195

[tik-stbuehler]

For consistency (with labelStyle and labelType) you probably should name the label class property labelClass instead of className.

(And I have no idea why you had to redo the way the class is applied - didn't classed work with the calculated class list?)

[kaelhem]

Hello. I changed the property name for labelClass as you asked. I replaced the "classed" stuff that allow to add/remove a specific class name to be able to specify several classes in the className parameter (like this : customClass1 customClass2).

If you think there is a better way to do it, tell me I will update the PR.

[tik-stbuehler]

I'm not a maintainer, just a user (and I'm interested in the feature too) :)

According to the API doc classed should work with multiple classes (like "label customClass1 customClass2" - i.e. just pass labelClass instead of "label") - but I'm no d3 expert, so perhaps I'm missing something here.

[kaelhem]

ok :)
I will have look, not a d3 expert too :)

edit:
@tik-stbuehler You were right, I've update the PR.

[tbo47]

I ported the project to es6 and I accept contributions!
https://github.com/tbo47/dagre-es