[RFE] add option to customize iso with custom prm file

# Feature Request #

## Desired Feature ##

I have a full custom prm file. It would be great if users could feed custom prm files to `coreos-installer iso customize` directly instead of using the `--dest-karg-append` or `--dest-karg-delete` flags to overwrite kernel params, which is very tedious.

## Other Information ##
