Detect external changes and hot-reload markdown

[atifsyedali]

It would be really nice if we could hot-reload the markdown files as they are changed in an external editor. Would allow to use an external editor and the browser showing the rendered result side-by-side without the edit-switch-refresh workflow.

[sinedied]

I though about this and while it could be useful indeed, I initially wanted to keep the dependencies down a bit (detecting FS changes comes with some weight). My intent was to implement at some point side-by-side live edition, which would be simpler workflow-wise.