feat: Standalone binary with `--input` flag

Is your feature request related to a problem? Please describe. Quartz requires cloning a repo and putting your Markdown files inside of it. This doesn't really fit with my workflow.

Describe the solution you'd like I would like a single binary that simply converts a given folder of Markdown into a static site.

Something like this:

quartz --input ~/notes/ --output ./dist

Describe alternatives you've considered Using alternatives to Quartz

Thanks!