Support stop send 0 flow counters

[wbchn]

This plugin still send count: 0 after stop writing events:

2016-10-11 06:41:01 +0000 fluentd-metrics: {"count":0,"bytes":0,"count_rate":0.0,"bytes_rate":0.0,"tag":"debug.where"}

so, we added a param delete_idle, same as deleteIdleStats in statsd:

  deleteIdleStats:  don't send values to graphite for inactive counters, sets, gauges, or timers
                    as opposed to sending 0.  For gauges, this unsets the gauge (instead of sending
                    the previous value). Can be individually overriden. [default: false]

After setting delete_idle true, plugin send no flow counter after stop write events.

[tagomoris]

Looks good to add. I updated .travis.yml to remove 2.0.0.

[wbchn]

Thank you. @tagomoris