Add an interactive CLI to `npm start`

Created by: keyz

Implements #55 (closed).

Known issues:

• opn only opens applications (like Atom.app) but not binary executables in $PATH. To be specific, an user can't use edit vim unless there exists a Vim.app. To support that we'll need to use something like exec that picks up stuff from $PATH.

Demo:

Available commands: help, open (o), edit (e), verbose (v), exit

  Commands:

    help [command...]  Provides help for a given command.
    exit               Exits application.
    open               Opens your app in browser.
    edit [editor...]   Opens your app in editor. Defaults to `$EDITOR`; pass an extra string to specify your editor of choice
                       (e.g., `edit sublime text` or `e visual studio code`).
    verbose            Verbose mode (show all webpack logs).

cc @vjeux @gaearon