Support injected fs (webpack loader/memory-fs)

It would be nice to understand how to support a context where use of the fs module is injected, i.e. in a [webpack loader](https://webpack.js.org/api/loaders/#this-fs). I would like to be able to use this library in [`graphql-loader`](https://github.com/stephen/graphql-loader), but as-is this library accesses the fs module directly.

Start by tracing where the library accesses the fs module directly, then read webpack loader documentation for the injected this-fs context and inspect graphql-loader's usage. Done means the library can use an injected filesystem in that context without relying on the direct fs module, and graphql-loader can use it successfully.

Written by the indexing model from the issue text.