Allow stack new to download templates over HTTP(S)

[reinh]

If the template name is a HTTP(S) URI, it should be downloaded, e.g.:

stack new my-package http://example.com/template.hsfiles

This would allow people to publish and use templates without requiring a local checkout or inclusion in the stack-templates repo.

I'll give this a shot and open up a pull request as well.

[mgsloan]

Makes sense to me!