Markdown diagrams are not detected correctly

[mzhubail]

To Reproduce

Using this document

:`:mermaid
graph TD
..
`:`

You get

$ ./src/cli.js -i tmp.md -o test.png

Found 1 mermaid charts in Markdown input.

This is not a realistic case, but I think we should use an implementation that doesn't have unexpected behavior. I can try out using remark as mentioned here.