Feature: allow mmdc to parse Markdown files

[AlexanderWillner]

Is your feature request related to a problem? Please describe.
I'm always frustrated when editing a Mermaid diagram in VSCode in a Markdown file and I can't export it easily via CLI using mmdc.

Describe the solution you'd like
I'd like mmdc -i diagram.md to produce diagram.md.svg.

Describe alternatives you've considered
One can copy and paste the diagram to a new file without the back ticks, but this is unpleasant.

Additional context
In case more than one diagram is found in the file, multiple files could be generated.

[MindaugasLaganeckas]

@knsv: do you agree with the feature described above? I see it as a great help to our users, but conceptually md files are not mmd files.

[knsv]

This makes total sense. There are some things to consider though.

I assume that the markdown file would use markdown formatting and that the diagram would come in a code chunk like this one:

```mermaid
flowchart
   A-->B
```

What do we do if, or rather when, there are multiple diagrams in the markdown file?

My take would be to:

• When the input file is a mmd file, it should work as today
• When the input file is a md file, the new functionality should kick in
• Only mermaid code blocks are handled
• When there are multiple code blocks the results will be extracted using a count creating diagram.md-1.svg ... diagram.md-1.svg

I think this would be a very strong feature. It would increase the various situations where the cli tool could be used significantly.