new rule: portability

[boneskull]

This rule would check that--unless specified in the os field of package.json--the package is not portable as installed.

This would include things like:

• bash-isms, path expansion or shell scripts in postinstall
• OS-specific packages in dependencies or npm-shrinkwrap.json

This was prompted by a package which published an npm-shrinkwrap.json containing a OS-specific package.