Adapting log display format

You may display walt logs in a custom format by using:

$ walt log show --format FORMAT_STRING [other_options...]

FORMAT_STRING should be a python-compatible string formating pattern. The default is: '{timestamp:%H:%M:%S.%f} {issuer}.{stream} -> {line}'

A useful example in case of batch processing is to use '{timestamp:%s.%f}' for the timestamp part. It will be displayed as a POSIX timestamp (i.e. a float number of seconds since Jan 1, 1970).