Add a `--convert-to=...` option to `ronin-payloads build`

Now that various Payload classes define `to_*` methods, `ronin-payloads build` needs a `--convert-to=perl|php|python|ruby|nodejs|html|command` option which calls `.to_perl`/`.to_php`/`.to_python`/`.to_ruby`/`.to_node_js`/`.to_html`/`.to_command` on the `@payload` object, if it defines that method.