Add watch mode build/deploy

[rootasjey]

It'd great to add a watch mode to build and deploy an app/clock face without needing to type:

• fitbit$ build
• fitbit$ install

I imagine the command could be fitbit$ watch to watch the currently directory and necessary files (like *.js, .css, .gui and excluding node_modules).

Currently I've to execute these two commands for every changes which make the development workflow not smooth (tbh it slow me down a lot to build more clock face).

PS: I already saw that there's a PR to combine build and install which is a nice improvement.