- Use bash for `set -o pipefail`
- Fall back to simpler find command when the original command failed

Related: #1061

Instead of notifying the event coordinator (EventBox) whenever a new
line is arrived, start a background goroutine that periodically does the
task. Atomic.StoreInt32 is much cheaper than mutex synchronization
that happens during EventBox update.

    > time cat /tmp/list | fzf-0.10.1-darwin_amd64 --ansi -fqwerty > /dev/null

    real    0m4.364s
    user    0m8.231s
    sys     0m0.820s

    > time cat /tmp/list | fzf --ansi -fqwerty > /dev/null

    real    0m4.624s
    user    0m5.755s
    sys     0m0.732s

    > wc -l /tmp/list2
     2594098 /tmp/list2

    > time cat /tmp/list2 | fzf-0.10.1-darwin_amd64 -fqwerty > /dev/null

    real    0m5.418s
    user    0m10.990s
    sys     0m1.302s

    > time cat /tmp/list2 | fzf-head -fqwerty > /dev/null

    real    0m4.862s
    user    0m6.619s
    sys     0m0.982s