support of --loglevel for "build"

[raDiesle]

Is your proposal related to a problem?

• the current output of react-scripts build can be very verbose.
  This is especial true if our CI server nodes are under heavy load, so the status of webpack % is shown many many lines in the logs. This is no useful information at all.

Describe the solution you'd like

support of react-scripts --loglevel=warn, which will output only warn or error level

Describe alternatives you've considered

• ignore it