Added support for buffers. Fixes #4

As with #4, I have a need to support loading from in-memory images in some `spritesmith` tests. This PR addresses that by adding `Buffer` support. In this PR:

- Added `Buffer` support (requires `type` specification)
- Added test for buffers
`brfs` doesn't support `path.join`. As a result, we need to use string concatenation for this filepath

substack/brfs#32