Add CLI option for watcher polling interval

In some environments the fs watcher falls back to polling for
changes. Both our current watcher and the proposed new watcher
both had support for user-defined polling intervals. I see no
reason not to add this for users who want extra control.

Fixes sass#801.

bin/node-sass

@@ -47,6 +47,7 @@ var cli = meow({
     '  --source-map-root          Base path, will be emitted in source-map as is',
     '  --include-path             Path to look for imported files',
     '  --follow                   Follow symlinked directories',
+    '  --poll-interval            The filesystem polling interval in ms when the watcher has to poll',
     '  --precision                The amount of precision allowed in decimal numbers',
     '  --importer                 Path to .js file containing custom importer',
     '  --functions                Path to .js file containing custom functions',
@@ -240,7 +241,11 @@ function watch(options, emitter) {
     watch.push(i);
   }
 
-  var gaze = new Gaze();
+  if (options.pollInterval && typeof options.pollInterval !== 'number') {
+    emitter.emit('error', 'The --poll-interval must be an integer.');
+  }
+
+  var gaze = new Gaze(null, { interval: options.pollInterval });
   gaze.add(watch);
   gaze.on('error', emitter.emit.bind(emitter, 'error'));
 

test/cli.js

@@ -11,7 +11,7 @@ var assert = require('assert'),
     LIBSASS_VERSION = null;
 
 describe('cli', function() {
-  
+
   before(function(done) {
       var bin = spawn(cli, ['-v']);
       bin.stdout.setEncoding('utf8');
@@ -235,6 +235,21 @@ describe('cli', function() {
       }, 100);
     });
 
+    it.only('should error when watching and --poll-interval is not an integer', function(done) {
+      var src = fixture('simple/tmp.scss');
+      var bin = spawn(cli, ['--watch', src]);
+      var didEmit = false;
+
+      bin.stderr.once('data', function() {
+        didEmit = true;
+      });
+
+      bin.once('close', function() {
+        assert.equal(didEmit, true);
+        done();
+      });
+    });
+
     it('should emit `warn` on file change when using --watch option', function(done) {
       var src = fixture('simple/tmp.scss');