Add multiple options to the logs plugin

New version of the logs plugin broke compatibility (as reported in #1534).
So options have been added to handle display:
* -t/--tail to tail logs (already existed)
* -n/--num NUM to display only this number of lines
* -p/--ps PS to display logs only from provided process
* -q/--quiet to display raw logs without color, time and names

This should close #1534

plugins/logs/functions

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+usage() {
+  echo "Usage: dokku logs <app>"
+  echo " display recent log output"
+  echo ""
+  echo " -n, --num NUM        # the number of lines to display"
+  echo " -p, --ps PS          # only display logs from the given process"
+  echo " -t, --tail           # continually stream logs"
+  echo " -q, --quiet          # display raw logs without colors, time and names"
+}