subcommand restructure and bashstyle refactor. closes #1579

plugins/plugin/subcommands/update

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+plugin_update_cmd() {
+  local desc="updates plugin to optional committish and calls update plugin trigger via command line"
+  local cmd="plugin:update"
+  if [[ -n "$2" ]]; then
+    local PLUGIN="$2"
+    local PLUGIN_COMMITTISH="$3"
+    plugn update "$PLUGIN" "$PLUGIN_COMMITTISH"
+  fi
+  plugn trigger update
+}
+
+plugin_update_cmd "$@"