Added mermaid support as an extra module

[knsv]

Thanks for a good documentation system! A very good blend between flexibility, simplicity and structure. I am looking at using docker in some projects but as I like to have mermaid for graphs in the dokumentation I decided to add that.

This pull request is based on pull request #79 and uses marked to generate the mermaid code. In order to keep things simple I added the mermaid files in the extras hierarchy.

One might consider adding a build step to get fetch the extras from npm instead of having them checked in into the repository.

[knsv]

I also added a fix for the headings panel as that functionality must have broken in the switch to marked. I realize there are probably more code that could be removed but leave that for later commits... :)

[alinex]

I won't change the tripple back quotes syntax for this because it should be used to highlight code, so in my meaning your syntax should highlight the mermaid syntax. To overcome this I added the alternative block with three dollar signs and an implementation name to execute the code within.