Limit the size of files that IPFS will fetch from the P2P network.

The IPFS api is nice and simple, but can cause problems for people wanting to use it as a gateway to serve images. A request for the wrong hash could cause ipfsd to start fetching a ton of data that is not actually desired.

The solutions I have at the moment are to do `ipfs file ls` first and then fetch, but that requires me to set up my own middleware when it would be nice to have the daemon fetch it for me.