Add optional mermaid shortcode

[Rihoj]

Issue

Fixes #550

Description

Adds support for Mermaid-js

Warning

Styling has not been worked out, and there may be some odd looks. Check the linked issue #550 for an example.

[hossainemruz]

@Rihoj Updated the shortcode. Now, it uses page level configuration instead of global mermaid configuration. In order to enable mermaid for a page, you have to set mermaid: true in the page frontmatter. Now, mermaid script will load only in those pages.

Regarding the style issue of Git graph. It seems to be an issue of mermaid-js itself. We have no control of the generated svg. We can use custom theme but it will be lot of hassle. So, I have added an optional background parameter to the shortcode. User can set this to black when there is any readability issue.

Let me know, what you think.