Adding ability to skip processing remotes

Since 6to5 prepends all scripts with `"use strict";`, I was unfortunately bitten by a remote dep that used an undeclared variable by mistake and broke a feature in my application.

This adds a `onlyLocals` option to the plugin, which will allow it to bypass remote dependencies. I've also added a new test for this, which included some re-arranging in the `test` dir.

In order to maintain backwards-compatibility, I've made this particular behavior opt-in. However, I think it's probably safer to only process locals by default. As such, I'd like to request that this be made the default behavior and users can opt in to processing remotes with a different flag. (eg: `processRemotes: true`)