Support added to read user and password also from file

Hi,
great script!

Since username and password had to be supplied via arguments to the script they showed up in command history, process lists and such. To increase security, options have been added to take these values from files instead.

Command line supplied values take precedence over values in files. So if e.g. username is supplied via --user and the new argument --userfile it will not take it from the file.
--userfile and --pwfile (and not --passfile) were used in order not to break --pass.

If you like it, feel free to pull.
Cheers!