add NODE_EXTERN to node::Start

[joelrbrandt]

@bnoordhuis
(same as #3376 but with different destination branch)

I'm building node as a DLL on Windows (which works awesome by the way) for embedding inside another application.

node::Start isn't currently exported. It's a useful function.

Thanks!

[bnoordhuis]

Thanks Joel, landed in b9c5eee.

[Kosta-Github]

@bnoordhuis would you also be willing to add something like this joelrbrandt@b903e8a in order to support the creation of DLLs (windows) and shared libs (Linux/OSX) as well? I am also thinking about embedding node into an existing hosting c++ application for providing scripting functionality...

[bnoordhuis]

@Kosta-Github That commit in itself is not enough to produce a working library but see #7336.