Adds extension file that should make usage on windows easier

After talking to some people on IRC and getting pointed to a guide on installing platform specific code this is what I came up with.

The alternative is to create another gem called maybe `shopify_theme_windows` or something, but I personally thing that's super ghetto. I'd much rather have us use this approach. There is a concern that people might be installing this as root and it can result in running arbitrary code.

I haven't been able to test this out on a Windows box yet, since I don't have one correctly configured on me right now. I can verify this works tonight or something, when I have access to one.

If this works it'll fix #46 

Please review @maartenvg @jduff 

/cc @autonomatt @Kottmaskinen since this might be relevant to your interests