Add --enable for Safari 11

[gauntface]

This is the driver than needs this:

$ safaridriver
Could not start server: must specify at least one configuration argument.

Usage: safaridriver [options]
    -h, --help                Prints out this usage information.
    -p, --port                Port number the driver should use. If the server
                              is already running, the port cannot be changed.
                              If port 0 is specified, a default port will be used.
    --enable                  Applies configuration changes so that subsequent WebDriver
                              sessions will run without further authentication.

[jeffposnick]

This should be fixed by #120.