Large file support

While reading the source, I noticed that a number of commands, at least `head` and `cat` when given a file argument, read in the entire file in memory before doing anything else. This may work fine for small files, but if you want to take the oldest few lines from a HUGE logfile, for example, or when wanting a few bytes from /dev/urandom, this will create a performance problem. Might be related to the `cat` hangup symptom [here](https://github.com/dthree/cash/issues/31#issuecomment-189788466). (^C problem might or might not be related?)
I also noticed that a command gets its entire stdin input from the calling function (presumably vorpal); same argument holds.

I've not had the chance to test the actual behaviour of the program as I don't have a desktop nearby at the moment, so sorry for that; but if I'm not mistaken, per-line buffering might be a good idea.

Last but not least: great project, keep up the good work! :)