Exclude containers by label.
Exclude containers by label.

Closes #159

lytics/logspout,jbachus/logspout,lytics/logspout,saiwl/logspout,jacobdr/logspout,jacobdr/logspout,bn0ir/logspout,gliderlabs/logspout,gliderlabs/logspout,saiwl/logspout,bn0ir/logspout,jbachus/logspout