packetbeat: add desctiption to -I option
packetbeat: add desctiption to -I option

the usage of '-I' option is not clear. Provide a meaningful
description, which will be shown with '-h'.

Signed-off-by: Tigran Mkrtchyan <d922616d71ca96f778c63ea49015b141afa0a2ae@desy.de>

yapdns/yapdnsbeat,yapdns/yapdnsbeat