Improves first constructor argument

This closes #21.

DOCUMENTATION.md

@@ -73,6 +73,12 @@ let camera = new Instacam(
 );
 ```
 
+You can also **pass a selector** to the constructor, and Instacam will get the DOM element for you:
+
+```js
+let camera = new Instacam('#canvas1');
+```
+
 **Custom options** are passed through the second parameter:
 
 ```js

package.json

@@ -1,6 +1,6 @@
 {
   "name": "instacam",
-  "version": "1.0.2",
+  "version": "1.1.2",
   "description": "Instant canvas video",
   "main": "src/instacam.js",
   "scripts": {

src/instacam.js

@@ -7,7 +7,7 @@ export class Instacam {
 
   /**
     Class constructor
-    @param {Object} viewport - canvas element from the DOM
+    @param {Object} viewport - selector or canvas element from the DOM
     @param {Object} options - custom options of the class
   */
   constructor(viewport, options) {
@@ -24,9 +24,14 @@ export class Instacam {
       return;
     }
 
+    // rewrites the viewport element if the user passed a selector to the constructor
+    if (typeof viewport === 'string') {
+      viewport = document.querySelector(viewport);
+    }
+
     // checks the viewport element
     if (typeof viewport === 'undefined' || viewport === null || viewport.nodeName.toLowerCase() !== 'canvas') {
-      throw new Error('Invalid viewport, you need to pass a valid HTML5 canvas element');
+      throw new Error('Invalid viewport, you need to pass a valid selector or HTML5 canvas element');
     }
 
     // initializes the viewport