Add watch mode for glow

[Dangerise]

Is your feature request related to a problem? Please describe. I'm using helix editor on my terminal and I need to preview my markdown file

Describe the solution you'd like Add a watch mode for glow, when the glow is watching a file, glow reload the file when the file is edited, and jump to the position which is edited.

[inifares23lab]

opened a PR for this. it is not feature complete yet, but it works.