Allow username/password to be provided in configuration

[WattyRev]

It seems like a nice option to be able to pass in a username/password instead of putting it in a .ftppass file. With this change, you could do something like:

grunt ftpush --my-username=watty --my-password=hunter2

With a configuration of

ftpush: {
            build: {
                auth: {
                    host: "myhost",
                    port: 21,
                    username: grunt.option("my-username"),
                    password: grunt.option("my-password")
                },
                ...
            }
        }

This change does not impact the previous key process, but will override that process if both username and password are provided.