Add option to read username/password from file

[ckastner]

This fixes #575 by adding an option --authfile. The argument is expected to be a filename with the following exact syntax:

username:foo
password:password123

In spirit with --url, --username and --password checks, it is a conflict when --authfile is used when any of these other options are used.

[alanxz]

Please run clang-format on tools/common.c, which should resolve the check related to formatting.