append graph rather than replace, add imageDir option

[bgschiller]

My use case may be a little different. I have a markdown doc that will be actively maintained, but mostly only viewed on github. I want to include mermaid graphs, but they will continue to be changed over time. My goals:

• apply changes inline (no separate dist folder with compiled markdown)
• keep a copy of the mermaid source that produced a diagram alongside the diagram
• avoid muddying up the docs directory with a whole bunch of images

To that end, I created a fork that introduces the following options:

1. A plugin-level imageDir option that specifies where to put svgs, relative to the file including them.
2. A .comment modifier (perhaps .summary would be a better name?) that maintains a copy of the mermaid markup in the file. This is easier to understand if you can see it. It changes:

    ```mermaid.comment
    sequenceDiagram
        Alice ->> Bob: Hello Bob, how are you?
        Bob-->>John: How about you John?
    ```

into

![](sha1here.svg "`mermaid` image")


<details><summary>Mermaid source</summary>

    ```mermaid.comment
    sequenceDiagram
        Alice ->> Bob: Hello Bob, how are you?
        Bob-->>John: How about you John?
    ```

</details>

It is smart enough to not continue to apply that transformation on subsequent runs, and to update the image when the mermaid.comment block changes.

3. Finally, I added an .inline modifier, that turns images into base64 strings and uses a data:image/svg+xml;base64,... data-url instead of creating a file. But it turns out Github won't render inline svgs, so I'm not actually using that option.

If you'd like to include any of these, I'd be happy to prepare a PR. I have added tests for all the cases. In debugging my changes, there were several of them that would have been caught immediately by using typescript. If that's something you're open to, I'd also be happy to prepare a PR switching over to TS.