Add support for displaying images inside nodes

[yuruotong1]

My dot is:

digraph structs { A [label="mmm" image="https://iconsapi.com/5ee261ede4b0b788a932a2eb.svg" shape=none]; }

The result as follow.

I don't think it's right

[magjac]

Sorry for the delay. This is a known limitation. I haven't figured out how create a good user interface to support this, given that images needs to be explicitly added using the underlying library [d3-graphviz addImage())(https://github.com/magjac/d3-graphviz#images) method.

[magjac]

I took the liberty to rephrase the summary of this issue to reflect that it's an enhancement proposal.