Add command line parameters for logging

I added logging parameters, "threshold" and "log_dir."

- log_dir specifies the directory to flush logs
- threshold specifies the logging level

The following is a example of the parameters.

    bin/walter -c tests/fixtures/pipeline.yml --threshold=Info --log_dir=tmp 

Why not use `!` operator? cf) http://play.golang.org/p/MbPc5NtwzA