Add allow-require option to no-commonjs rule

This fixes #548

docs/rules/no-commonjs.md

@@ -21,6 +21,24 @@ module.exports = { a: "b" }
 exports.c = "d"
 ```
 
+### Allow require
+
+If `allow-require` is provided as an option, `require` calls are valid:
+
+```js
+/*eslint no-commonjs: [2, "allow-require"]*/
+
+if (typeof window !== "undefined") {
+  require('that-ugly-thing');
+}
+```
+
+but `module.exports` is reported as usual.
+
+This is useful for conditional requires.
+
+### Allow primitive modules
+
 If `allow-primitive-modules` is provided as an option, the following is valid:
 
 ```js

src/rules/no-commonjs.js

@@ -12,6 +12,11 @@ function allowPrimitive(node, context) {
   return (node.parent.right.type !== 'ObjectExpression')
 }
 
+function allowRequire(node, context) {
+  if (context.options.indexOf('allow-require') < 0) return false
+  return true
+}
+
 //------------------------------------------------------------------------------
 // Rule Definition
 //------------------------------------------------------------------------------
@@ -52,6 +57,8 @@ module.exports = {
         if (module.type !== 'Literal') return
         if (typeof module.value !== 'string') return
 
+        if (allowRequire(call, context)) return
+
         // keeping it simple: all 1-string-arg `require` calls are reported
         context.report({
           node: call.callee,

tests/src/rules/no-commonjs.js

@@ -27,6 +27,7 @@ ruleTester.run('no-commonjs', require('rules/no-commonjs'), {
     { code: "var bar = proxyquire('./bar');" },
     { code: "var bar = require('./ba' + 'r');" },
     { code: 'var zero = require(0);' },
+    { code: 'require("x")', options: ['allow-require'] },
 
     { code: 'module.exports = function () {}', options: ['allow-primitive-modules'] },
     { code: 'module.exports = "foo"', options: ['allow-primitive-modules'] },