Add --verbose flag to control log level

[tpwo]

With a new approach in logging introduced in 334330f, it will be a good idea to allow for easy modification of the log level without changing the source code.

I like the idea with count action in argparse which literally counts how many times a flag was passed. A simple proposal:

pyzet show
# 0 times, WARNING log level (because we sometimes show warnings to the user)
pyzet -v show
# 1 time, INFO log level
pyzet -vv show
# 2 times, DEBUG log level
pyzet -vvv show
# 3 times, still DEBUG log level because we don't want to go further