Close log file during teardown + some code cleanup

Fixes #815 (closed)

It also contains some clean up. Main change:

Close log file during teardown

config.ConfigureLogger() now returns *os:File. The log file gets closed
during teardown (Bitmask.Close()).
cli (cmd/bitmask-helper/main.go and) and helpers (pkg/pickle/helpers.go)
were also fixed.