Only use Asset option for the inner file, while leaving the base file in tact.

Fixes #29.
All tests still pass. There are other ways of doing what I wanted to do, but this one was the simplest and doesn't require introducing any new methods. 

Basically, I changed readFiles to only use opt.Asset for the inner file. The base file is still parsed normally if provided. If you want to use opt.Asset without providing a base file, you can just pass in a blank string for the base path.

Let me know what you think.