feature: ability to output mermaid/dot/graphviz dependency graphs

[ssbarnea]

Mermaid is an open-souce graph maker that can easily be integrated in any web page and also with documentation build tools (sphinx, markdown based tools,...)

Example of graph:

graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

Note: there is also a feature request to enable mermaid markdown extension on github, which would render it directly.

[gaborbernat]

PR welcome.

[kdeldycke]

dot and graphviz are already implemented. I proposed a PR which implements Mermaid output at: https://github.com/tox-dev/pipdeptree/pull/195

If this PR is merged upstream, them this issue can be closed.