Support multiple queues via --queue

[lox]

This allows comma delimited queues to be passed in to the --queue filter for forthcoming elastic stack changes.

[sj26]

Sadly this seems to have broken the latest buildkite-metrics. Requesting metrics without any queues specified returns 0 cloudwatch metrics:

$ ./buildkite-metrics-Linux-x86_64-v1.2.0-12-gecec585 -org ... -token ...
2016/12/05 23:49:26 Collecting running and scheduled build and job metrics
...
2016/12/05 23:49:27 Extracted 199 cloudwatch metrics from results
...

$ ./buildkite-metrics-Linux-x86_64-v1.2.0-14-g2765755 -org ... -token ...
2016/12/05 23:50:02 Collecting running and scheduled build and job metrics
2016/12/05 23:50:03 Builds page 1 has 0 builds, next page is 0
2016/12/05 23:50:03 Collecting agent metrics
2016/12/05 23:50:04 Agents page 1 has 6 agents, next page is 0
2016/12/05 23:50:04 Extracted 0 cloudwatch metrics from results
2016/12/05 23:50:04 Finished in 1.25458307s