utils: add usage summary generator

Credit: @watilde
Reviewed-By: @iarna
PR-URL: npm/npm#12485

lib/utils/usage.js

@@ -0,0 +1,27 @@
+'use strict'
+var aliases = require('../config/cmd-list').aliases
+
+module.exports = function usage (cmd, txt, opt) {
+  var post = Object.keys(aliases).reduce(function (p, c) {
+    var val = aliases[c]
+    if (val !== cmd) return p
+    return p.concat(c)
+  }, [])
+
+  if (opt || post.length > 0) txt += '\n\n'
+
+  if (post.length === 1) {
+    txt += 'aliase: '
+    txt += post.join(', ')
+  } else if (post.length > 1) {
+    txt += 'aliases: '
+    txt += post.join(', ')
+  }
+
+  if (opt) {
+    if (post.length > 0) txt += '\n'
+    txt += 'common options: ' + opt
+  }
+
+  return txt
+}