Add position support as per ECMA-262 spec; Fixes gearcase#3

.gitignore

@@ -4,3 +4,4 @@ npm-debug.log*
 
 /node_modules
 /test/coverage
+yarn.lock

index.js

@@ -1,6 +1,6 @@
 'use strict';
 
-module.exports = function (str, prefix) {
+module.exports = function (str, prefix, pos) {
 
   if (typeof prefix === 'undefined' || prefix === null) {
     return false;
@@ -9,11 +9,23 @@ module.exports = function (str, prefix) {
   str    = String(str);
   prefix = String(prefix);
 
+  if(pos === undefined || pos < 0) {
+    pos = 0;
+  }
+  else {
+    pos = +pos || 0;
+  }
+
   var l = prefix.length;
-  var i = -1;
+  var i = pos - 1;
+  var j = 0;
+
+  if (i > str.length) {
+    return false;
+  }
 
   while (++i < l) {
-    if (str.charAt(i) !== prefix.charAt(i)) {
+    if (str.charAt(i) !== prefix.charAt(j++)) {
       return false;
     }
   }

test/spec/index.js

@@ -41,4 +41,24 @@ describe('start-with: ', function () {
   it('startWith(\'abc\', \'bc\') => false', function () {
     expect(startWith('abc', 'bc')).to.equal(false);
   });
+
+  it('startWith(\'abc\', \'bc\', -1) => false', function () {
+    expect(startWith('abc', 'abc', -1)).to.equal(true);
+  });
+
+  it('startWith(\'abc\', \'bc\', 1) => false', function () {
+    expect(startWith('abc', 'bc', 1)).to.equal(true);
+  });
+
+  it('startWith(\'abc\', \'abc\', 10) => false', function () {
+    expect(startWith('abc', 'abc', 10)).to.equal(false);
+  });
+
+  it('startWith(\'abc\', \'abc\', \'foo\') => false', function () {
+    expect(startWith('abc', 'abc', 'foo')).to.equal(true);
+  });
+
+  it('startWith(\'abc\', \'abcd\') => false', function () {
+    expect(startWith('abc', 'abcd')).to.equal(false);
+  });
 });