Fix: lttng-live flush when output to file

Fix: lttng-live flush when output to file

When the standard output is redirected to a file, we need to flush the
output between each packet, otherwise the stream buffering may keep us
from seeing events for a while in a low-throughput trace.

While we are there, document that lttng-live does now support the
--output option in babeltrace 1.x.

Fixes #1010

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>