[appserver-io/webserver] Backreference based condition checking config wide

[wick-ed]

By now only the rewrite module does support backreference based condition checking such as this. The usage of backreferences and the connected opportunity to check for several factors when triggering configuration details such as proxy forwarding or access restrictions.

UAC:

• There MUST be basic backreferences available to check for in configuration conditions
• The syntax of all condition fields MUST be the same
• Condition checking MIGHT be implemented within its won library