Add flow typechecking as a webpack plugin

Created by: rricard

This will only run if a @flow annotation is seen in an user's file:

• At the first @flow file found:
  • Write a .flowconfig if none exists
  • Run flow-typed install
  • Add some custom flow-typed installs (implicit jest behind react-scripts for instance)
• When a file with an @flow comment changes during a compilation:
  • Run a flow check
  • If there are some type errors in flow: show warnings
  • If flow itself crashes: show error
• Refactor

Followup goals:

• Return-code-based success of the command (instead of looking for "No Errors!"
• Detect if a global flow installation exists, if yes, check its version, if not equal to CRA's version, show a warning that compilation may be slowed down

• Ensure the first flow status run was done after complete init (especially flow-typed)
• Add flow-typed dir to .gitignore during the first run
• Add to end-to-end testing

Followup of a discussion in #72 (closed)