Mermaid integration

[xiaozongyang]

Background

mermaid.js is a fashion plugin for rending diagrams from source code in markdown syntax.

You may use the following code to rending a flowchart, this example is from here.

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

So the motivation is to enhance hexo-theme-pure by integrating https://github.com/webappdevelp/hexo-filter-mermaid-diagrams. The code modification is referred to https://github.com/webappdevelp/hexo-filter-mermaid-diagrams

[xiaozongyang]

ping @cofess