add :trail option for starting the cluster in... well, trailing mode.

 - the current implementation assumes that topics should be read from the
   starts but we have use cases where we only cares about what comes
   next (especially for topics where we keep messages for long periods
   of time)
 - initially wanted to support an :offset option butit became too messy too
   quickly
Merge branch 'master' of https://github.com/davideauee/poseidon_cluster into trailing
Merge pull request #1 from davideauee/trailing

add :trail option for starting the cluster in... well, trailing mode.