Add support for pass/drop/tagpass/tagdrop for outputs

Reuses same logic as the plugins for filtering points, should be only
a marginal performance decrease to check all the points before writing
to the output.

Added examples to the README as well (for generic pass/drop as well as
output pass/drop/tagpass/tagdrop).

X-Github-Closes #398

closes #398
closes #401

accumulator.go

@@ -107,7 +107,7 @@ func (ac *accumulator) AddFields(
 	}
 
 	if ac.pluginConfig != nil {
-		if !ac.pluginConfig.ShouldPass(measurement) || !ac.pluginConfig.ShouldTagsPass(tags) {
+		if !ac.pluginConfig.Filter.ShouldPass(measurement) || !ac.pluginConfig.Filter.ShouldTagsPass(tags) {
 			return
 		}
 	}