Provide easy configuration property to turn off greylisting, and document it in the README.md and reference.conf

[charlescapps]

I found that my emails were being greylisted unexpectedly, so I wanted to turn off the greylisting feature.

There's no easy config property to turn off greylisting, so I had to modify the code and recompile the scala.

There should be an easy prop you can set in application.conf, e.g.

sender {
    greylist {
        enabled = false
    }
}

[faust64]

Pushing PR ...