add $passenger_pre_start variable

[wyardley]

passenger_pre_start (unlike other passenger settings that are not directly in the module, and can be easily added with an 'append' directive inside the vhost) has to be outside the scope of the server {} block. If this is too specific, I could alternatively make a different append variable that appends in the vhost file but outside that block.

I believe it might be possible to put it in http {} also, but most examples show it following the server {} block, but outside of its scope.
https://www.phusionpassenger.com/library/config/nginx/reference/#passenger_pre_start

Since the required stdlib version still doesn't seem to have a validate_url function, I just validated that it was a string, rather than trying to come up with a regex that would validate all possible valid URLs.

I added a test, but didn't add a test for the actual validation of parameters (most other params don't seem to have this at present).