Pipe closing isn't handled gracefully #20

When loop is piped to some program that only takes a subset of loop's output the program prints throws a panic message that may confuse people when used as part of a long command chain.

Example:

user@pc $ loop 'echo $COUNT' | head -n 2
0
1
thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', libstd/io/stdio.rs:692:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.

The expected behaviour is that loop exits gracefully upon detecting the pipe is closed.