Use standard open command in current plugins

Substitutes the current duplicate logic for the standard
and cross-platform function open_command in plugins:

  frontend-search, jira, node, web-search

plugins/node/node.plugin.zsh

@@ -1,13 +1,5 @@
 # Open the node api for your current version to the optional section.
 # TODO: Make the section part easier to use.
 function node-docs {
-  # get the open command
-  local open_cmd
-  if [[ "$OSTYPE" = darwin* ]]; then
-    open_cmd='open'
-  else
-    open_cmd='xdg-open'
-  fi
-
-  $open_cmd "http://nodejs.org/docs/$(node --version)/api/all.html#all_$1"
+  open_command "http://nodejs.org/docs/$(node --version)/api/all.html#all_$1"
 }