Feature request: Support external variables

YARA rules can include [external variables](https://yara.readthedocs.io/en/v3.7.0/writingrules.html#external-variables), which `yara` allows you to define with a `-d` flag.

It would be great if `yextend` could support external variables (perhaps with the same `-d` command line switch) and pass them on to the underlying `yara` invocations