Add watch mode

[sarmong]

Is your feature request related to a problem? Please describe.

I have a go server with auth in front of html files generated by quartz. I want quartz to watch files for changes on the server and rebuild.
However, I don't want to run --serve, because I don't need local server and want to save my server resources.

Describe the solution you'd like
I added this feature for myself, as it seems to be just updating a few ifs.

I can open a PR if you are okay with adding it to upstream.

[aarnphm]

take a look at handlers.js, might worth to write a custom function for your workflow.

fwiw we might think of a Quartz API one day ;)

[sarmong]

@aarnphm I know, I mean I already added --watch flag for myself and asking if you want it upstream.

[saberzero1]

I suppose there could be a use case for rebuilding only.

Feel free to open a PR. I'll take a look.