Strict option to fix #174.

This is a backwards-compatible fix for #174 that would probably be preferable over #320. It requires passing options to, `exports({ strict: true })` to ensure no conflicting methods get added to `lodash` / `underscore`.

I think this seems like a good way of handling things, and the `strict` option could be documented such that it will always maintain compatibility with any enhancements added to `lodash` / `underscore`. For instance, if `count` was added to either, `underscore.string` would be updated to not export it (possibly causing some applications to require updates).