Add optional argument to getFileName

I'd like to use node-bindings in [node-pre-gyp](https://github.com/mapbox/node-pre-gyp).

In the case of node-pre-gyp, which packages binaries, the exact relative path is known to the versioned binary `.node` (so multiple tries are not needed). But node-pre-gyp allows complex versioning to be declared in a modules `package.json`. In order to only declare that versioning in one place (package.json and not the js require) it makes sense to provide a node-binding like require shortcut.

So `node-pre-gyp` needs to find out the path to the `package.json` in order to look up the versioning info (and module path). I found the `getFileName` and `getRoot` parts of node-bindings useful for this.

The one problem is that `getFileName` obviously resolves against the calling js file. In my case I want to call it from node-pre-gyp but actually resolve the file that is calling node-pre-gyp. This patch allows that. Let me know if you can think of better ways, but this works well.

For reference this is how I'm using it: https://github.com/mapbox/node-pre-gyp/compare/master...bindings.