support receiving a batch of asset paths

[ashleygwilliams]

currently the API receives a single origin_path for an asset and fetches it. we should also introduce an API that can receive multiple origin_paths at once, be it a path to a dir, a vector, etc. it's not clear to me if we should pick one way (an iter?) or support multiple ways, of doing this, but in general this logic would not add a ton of complexity/maintenance burden (which is also why it seems ok to leave it up to the user to iterate through assets...)