Adds support for custom GYP include

When developers have libraries in non-standard locations (or don't wish
to dump them into System32 on Windows), they need to run "node-gyp
configure", edit the generated files to add include paths, and then run
"node-gyp install"

This is doubly painful when you wanted to "npm install" something, but
it depends on a library being in a default path on your system.

I've added the ability for users to include an additional ".gyp" file in
all node-gyp builds by setting an environment variable
(NODE_GYP_ADDITIONAL_CONFIG), which they can use to configure
include/library paths.  No longer must I manually install packages or
dump files into global include directories.