[Texture] Add offset property to texture

Also add docs for both offset and repeat

Fixes #25

docs/src/internalComponents/texture.js

@@ -11,7 +11,12 @@ class texture extends DocInfo {
 
   getAttributesText() {
     return {
-      repeat: '',
+      repeat: 'Sets the [repeat property of the ' +
+      'Texture](http://threejs.org/docs/#Reference/Textures/Texture.repeat).\n\n' +
+      'Default: `(1, 1)`.',
+      offset: 'Sets the [offset property of the ' +
+      'Texture](http://threejs.org/docs/#Reference/Textures/Texture.offset).\n\n' +
+      'Default: `(0, 0)`.',
       wrapS: '',
       wrapT: '',
       anisotropy: '',

src/lib/descriptors/Material/TextureDescriptor.js

@@ -30,6 +30,19 @@ class TextureDescriptor extends THREEElementDescriptor {
       default: new THREE.Vector2(1, 1),
     });
 
+    this.hasProp('offset', {
+      type: propTypeInstanceOf(THREE.Vector2),
+      updateInitial: true,
+      update(threeObject, offset) {
+        if (offset) {
+          threeObject.offset.copy(offset);
+        } else {
+          threeObject.offset.set(0, 0);
+        }
+      },
+      default: new THREE.Vector2(0, 0),
+    });
+
     [
       'wrapS',
       'wrapT',