Allow for a 'simple' option

[brendo]

This plugin currently pulls in mermaid.cli, which brings in puppeteer, which installs in Chromium. This doesn't play nice in docker image which is based on Alpine.

The abstraction in mermaid.cli doesn't allow puppeteer's runtime settings to be altered, so while getting Chromium into the image is likely possible, we'd have no way to tell mermaid so that it can invoke puppeteer correctly.

Instead. Can we offer a simple mode in this plugin that simply takes:

```mermaid
graph LR
    Start --> Stop

And transforms it to be:

<div class='mermaid'>
graph LR
    Start --> Stop
</div>

This will rely on whatever is "generating the markdown" to ensure the mermaid JS file is available so that the graph can be generated client side, but it at least means that authors can start using the syntax now and the solution can be improved later.