pw_cli: Add argument to hide banner
pw_cli: Add argument to hide banner

This change adds an argument to the pw command which prevents it from
printing the Pigweed banner. This is useful when pw is called from
scripts to avoid polluting the output.

Bug: 143

Change-Id: I30af43bd7815aa0472a1cb1ac2ca3f5ef49976dc

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed