Cleanup & standardize config file

changes:

- -sample-config will now comment out all but a few default plugins.
- config file parse errors will output path to bad conf file.
- cleanup 80-char line-length and some other style issues.
- default package conf file will now have all plugins, but commented
  out.

closes #199
closes #944

plugins/inputs/mqtt_consumer/mqtt_consumer.go

@@ -78,7 +78,7 @@ var sampleConfig = `
   ## Use SSL but skip chain & host verification
   # insecure_skip_verify = false
 
-  ## Data format to consume. This can be "json", "influx" or "graphite"
+  ## Data format to consume.
   ## Each data format has it's own unique set of configuration options, read
   ## more about them here:
   ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md