FIX: Allow to use a plain javascript file for configuration.

Easier to write, easier to import, allows comments
and functions (for a foreseen next feature).
API: Configuration simplification.

- Allow to define function as "control" and "action" (utility functions are
  passed with the `context`).
- The default rule for a host is moved in the code.
- Callback function is optional for proxyWork.
API: Added `preserveHost` option in the configuration.

Copie the Apache ProxyPreserveHost directive :
By default, use the `host:port` value to give to the back end server
If `preserveHost==true` transmit the original value in the request to the proxy.
API: Simplified log function removing useless callback.

Logs additionaly the rule number used for the request (`##` if default)