Add new basicstats aggregator (#2167)

plugins/aggregators/basicstats/README.md

@@ -0,0 +1,43 @@
+# BasicStats Aggregator Plugin
+
+The BasicStats aggregator plugin give us count,max,min,mean,s2(variance), stdev for a set of values,
+emitting the aggregate every `period` seconds.
+
+### Configuration:
+
+```toml
+# Keep the aggregate basicstats of each metric passing through.
+[[aggregators.basicstats]]
+  ## General Aggregator Arguments:
+  ## The period on which to flush & clear the aggregator.
+  period = "30s"
+  ## If true, the original metric will be dropped by the
+  ## aggregator and will not get sent to the output plugins.
+  drop_original = false
+```
+
+### Measurements & Fields:
+
+- measurement1
+    - field1_count
+    - field1_max
+    - field1_min
+    - field1_mean
+    - field1_s2 (variance)
+    - field1_stdev (standard deviation)
+
+### Tags:
+
+No tags are applied by this aggregator.
+
+### Example Output:
+
+```
+$ telegraf --config telegraf.conf --quiet
+system,host=tars load1=1 1475583980000000000
+system,host=tars load1=1 1475583990000000000
+system,host=tars load1_count=2,load1_max=1,load1_min=1,load1_mean=1,load1_s2=0,load1_stdev=0 1475584010000000000
+system,host=tars load1=1 1475584020000000000
+system,host=tars load1=3 1475584030000000000
+system,host=tars load1_count=2,load1_max=3,load1_min=1,load1_mean=2,load1_s2=2,load1_stdev=1.414162 1475584010000000000
+```