Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

plugins/inputs/procstat/procstat.go

@@ -66,7 +66,7 @@ func (_ *Procstat) Description() string {
 func (p *Procstat) Gather(acc telegraf.Accumulator) error {
 	err := p.createProcesses()
 	if err != nil {
-		log.Printf("Error: procstat getting process, exe: [%s]	pidfile: [%s] pattern: [%s] user: [%s] %s",
+		log.Printf("E! Error: procstat getting process, exe: [%s] pidfile: [%s] pattern: [%s] user: [%s] %s",
 			p.Exe, p.PidFile, p.Pattern, p.User, err.Error())
 	} else {
 		for pid, proc := range p.pidmap {