Add configuration to append extra CLI flags to all generated commands

[forgedhallpass]

This could be useful if users would want to proxy all requests through Burp/Proxify. They could set, let's say the "Additional CLI flags" setting to -proxy http://localhost:8080.

Burp proxy configuration can be retrieved using:

callbacks.saveConfigAsJson("proxy.request_listeners")

The configuration could also appear under the CLI field, and should only apply to nuclei commands.