Apply CGI environment variables in the correct order

[syrusakbary]

This PR does the following:

• Allows receiving a --debug/-d flag to the run-unstable subcommand
• Removes the SCRIPT_NAME hack (it should be done by the package annotation)
• Prints the server where things are running (I got so lost before that I had no idea in which port it was running)

[Michael-F-Bryan]

I've updated this PR to remove the --debug flag. We actually enable the logger already at the start of the function - you can increase the global log level by adding more -v arguments (default is warn) and can also use $RUST_LOG for fine-tuning.

I'd prefer not to pass Config to the callbacks if we can. For a situation like this, the caller already knows the address we're serving from because they set up the runner, so we can just reuse it.

I also reorganised the order we set environment variables in so we'll apply the CGI variables first and give users the ability to override them. I felt that was better than https://github.com/wasmerio/wcgi/pull/37 because it means you still get a useful default when you don't $DOCUMENT_ROOT and $SCRIPT_FILENAME, rather than erroring out because mod_cgi.wasm doesn't know which script to invoke.

[theduke]

Looks good to me.