Add mermaid configuration

[ghost]

There should be ways that we can modify config of mermaid.

http://knsv.github.io/mermaid/#configuration35

mermaid.sequenceConfig = {
    diagramMarginX:50,
    diagramMarginY:10,
    boxTextMargin:5,
    noteMargin:10,
    messageMargin:35,
    mirrorActors:true
};

[JozoVilcek]

Yes, I thought about it before, but had no time to work out propagation of settings from gitbook and use them in plugin internals. I will be happy If you care to draft a pull request for such feature.