Automatic dependency loading

[groenroos]

As part of #175 and #189, some "core" functionality will be split out into separate dependencies.

The idea is that the developer who needs Uploads or Notifications would either use the CLI to add these capabilities, or just npm install them themselves (which is what the CLI is shorthand for). However both are cumbersome experiences from a "speed of development" perspective, as you'd have to move off from your IDE, exit Sapling, install, configure, then re-run Sapling and return to code.

Take a leaf from Laravel Mix's book, and have Sapling (in non-production mode) automatically install internal dependencies (and their npm dependencies) whenever they are called in code.