MDL-50115 grunt: provide a --root option to set base path

Windows users are not able to run grunt on a subpath due to platform
restrictions. So we provide this option to provide a workaround for that.

Thanks to Ty Delean who proposed this idea.

Gruntfile.js

@@ -26,8 +26,21 @@
 module.exports = function(grunt) {
     var path = require('path'),
         tasks = {},
-        cwd = process.env.PWD || process.cwd(),
-        inAMD = path.basename(cwd) == 'amd';
+        cwd = process.env.PWD || process.cwd();
+
+    // Windows users can't run grunt in a subdirectory, so allow them to set
+    // the root by passing --root=path/to/dir.
+    if (grunt.option('root')) {
+        var root = grunt.option('root');
+        if (grunt.file.exists(__dirname, root)) {
+            cwd = path.join(__dirname, root);
+            grunt.log.ok('Setting root to '+cwd);
+        } else {
+            grunt.fail.fatal('Setting root to '+root+' failed - path does not exist');
+        }
+    }
+
+    var inAMD = path.basename(cwd) == 'amd';
 
     // Globbing pattern for matching all AMD JS source files.
     var amdSrc = [inAMD ? cwd + '/src/*.js' : '**/amd/src/*.js'];