Allow ink function to run on each redacted field individually

[joe-pritchard]

I've made a slight modification so that the ink function is called each time a field is redacted instead of being called once when set (which would basically mean the developer had to run the entire redaction on their own copy of the data?)

I've also added a note about this option to the readme, updated the test, and taken the liberty of upgrading the supported php versions in composer.json