Added support for --with-http_addition_module

Turns out this is what I really needed for reverse proxies (node, python, and ruby apps mostly).

This commit adds the HTTP addition module for nginx as documented here http://nginx.org/en/docs/http/ngx_http_addition_module.html 

The command to add HTTP addition module is ```--with-addition``` in order to maintain uniformity with the other modules/module options.

Hope this helps someone in the future as it did for me.
Merge pull request #12 from durango/patch-2

Added support for --with-http_addition_module