Add ability to retroactively enable checksums

@ruebot, I'll let you update the screencap linked in the README. Also, coder is showing the following code style error"

```
Line 60: No key specified for array entry; first entry specifies key [sniffer_squiz_arrays_arraydeclaration_nokeyspecified]
    ),
```
Code looks good to me - know how to fix that?