inputs: pipe: rerun command when it terminates

This change causes the pipe input to keep rerunning the command.
Without this change, if the command crashes or simply stops sending
output for some unknown reason, then logging effectively stops.  Also,
it introduces a short delay before restarting the command to avoid
churning on a failing command.
Merge pull request #754 from jdve/rerun-pipe

inputs: pipe: rerun command when it terminates
Thanks for making Logstash more awesome!