Add support for watching a directory other than project root.

[Schmavery]

Love using this tiny simple server, thanks. I recently did this for myself and figured I'd share in case anyone happened to be interested.

The usecase is that I'm running a bundler/watcher already that's building my code as I edit, and if I watch both the src root and the bundled root the page refreshes twice for every edit (once for the actual edit, once for the bundle update). This way I can tell servor to only refresh when the bundle updates but not have to move around my index.html etc.

Maybe too specific to be included so feel free to close this if you think so.

[lukejacksonn]

Hey, glad you are liking servor and thanks for the suggestion! I'm trying to avoid adding more positional arguments still.. but this is the kind of thing that I'd consider putting in a config file which is probably going to be the next update here.

Happy to keep this PR open to see if others might find it useful 👍

[Schmavery]

Makes sense -- I first added it as a named arg but then realized I'd have to refactor the way the positional args work so this seemed like the only way to add it without any breaking changes.