head: unwrap IoResult before printing

    $ build/head README.md 
    Ok(uutils coreutils [![Build Status](https://travis-ci.org/uutils/coreutils.png?branch=master)](https://travis-ci.org/uutils/coreutils)
    )Ok(================
    )Ok(
    )Ok(uutils is an attempt at writing universal (as in cross-platform) CLI
    )Ok(utils in [Rust](http://rust-lang.org). This repo is to aggregate the GNU
    )Ok(coreutils rewrites.
    )Ok(
    )Ok(Why?
    )Ok(----
    )Ok(