Improve CLI (i.e. improve first usage experience)

[brillout]

As suggested by @tdfranklin the CLI could be improved in order to have a better first time usage experience.

Some ideas:

• Scaffold apps
  • Scaffold different types of apps
• Scaffold pages
• Scaffold other things?
• Make that plugins can add CLI commands
  • All scaffolding code could live in plugins

[brillout]

The relevant files for this ticket should mainly be these two:

• https://github.com/brillout/reframe/blob/master/core/cli/cli.js
• https://github.com/brillout/reframe/blob/master/core/utils/processReframeConfig.js

[brillout]

Done
Good job @tdfranklin