Add [defaults] config parsing for username/pw.

In some environments, there is a consistent username and password
that is not the vendor default.  Instead of hardcoding the site
specific credentials, allow for reading a defaults section out of
the config file so it overrides the vendor-specific default.
Merge pull request #13 from hcoyote/credential-defaults-in-cfg-file

Add [defaults] config parsing for username/pw.