add an option to disable mermaid

[PinkD]

always adding mermaid js file into template makes the generated html file bigger than 1MB

https://github.com/yzane/vscode-markdown-pdf/blob/4815dcc62d434cfe2bd9d81f4cf59bb3da339b65/template/template.html#L7-L9

[yzane]

Please upgrade Markdown PDF to ver1.4.4 and try.

[vowstar]

This change makes it impossible to use offline. It would be better if there is an option for offline use.