add proxy support and custom location values to nginx.  

fixes #901, fixes #916

Adding support for upstreams to nginx.  In addition I have also added support for the location_custom_cfg_prepend which allows for entering custom directives to the location.

I've also pre-plumbed some support for location_custom_cfg_append, however there are form requirements for that which are a little outside the scope of current functionality - i'll need to circle back to that.