mkdir: added verbose flag, changed error handling method

- The verbose flag needs more work in how it behaves with
  the -parents flag. To observe, compare output between this
  implementation and the actual mkdir, with both parent and
  verbose flags, and create a nested directory whose
  parent does not exist.
- When mkdir encounters errors, they are simply displayed.
  Any proceeding arguments are processed. Because the Fatal
  functions call os.Exit(1) and the Panic functions call
  panic after writing their log message, I removed usage of
  the log package and opted to simply display the error.