Don't rely on isTTY for reading from stdin. Fixes #451.

Actually isTTY is only relevant if we want to see whether we really have to read from stdin.

lib/cli.js

@@ -134,7 +134,7 @@ module.exports = function(args) {
   options.src = input[0] || null;
   options.dest = options.output || input[1] || null;
 
-  if (!options.dest && (process.stdout.isTTY || process.env.isTTY)) {
+  if (!options.dest && !options.stdout) {
     var suffix = '.css';
     if (/\.css$/.test(options.src)) {
       suffix = '';
@@ -142,18 +142,17 @@ module.exports = function(args) {
     options.dest = path.join(cwd, path.basename(options.src, '.scss') + suffix);
   }
 
-  if (process.stdin.isTTY || process.env.isTTY) {
-    if (!input.length) {
+  if (options.src) {
+    run(options, emitter);
+  } else if (!input.length && (process.stdin.isTTY || process.env.isTTY)) {
     console.error([
       'Provide a sass file to render',
       '',
-        'Example',
+      '  Example',
       '    node-sass --output-style compressed foobar.scss foobar.css',
       '    cat foobar.scss | node-sass --output-style compressed > foobar.css'
     ].join('\n'));
     process.exit(1);
-    }
-    run(options, emitter);
   } else {
     stdin(function(data) {
       options.data = data;