Get link by id

[fecorreiabr]

Hello!

Currently it's possible to get a link from the graph through several ways:

• using getNode and iterating over the links property
• via getLinks and getLink
• via nodes and links iterators

However, there's no way to get a link by its id property, which would be useful if you store these ids externally (when the graph is created or changed) for later use.

[fecorreiabr]

I believe a good solution would be to make getLink/hasLink find a link from an id if only the first argument is passed. I created a pr with a proposal for this.