CLI tools: allow rsync to be triggered automatically

Today, when developing changes against remote sandboxes like WoA or simple sites, 2 CLI commands are extremely useful:

- `jetpack watch`
- `jetpack rsync`

Ideally, I think it would be nice to be able to combine those 2, so one can develop locally and have their changes built and synchronized with their remote sandbox without having to run any command.

*******

Internal reference:
paYKcK-36O-p2#comment-2248

Start by locating the entry points for `jetpack watch` and `jetpack rsync` and reading how each command handles local changes and remote sandboxes. Define how automatic synchronization should combine their behavior, then verify that changes can be built and synchronized without manually invoking either command.

Written by the indexing model from the issue text.