Added srcset support (#18)

* added src set as a parsed attribute

* added tests

* changed to /a

* added multiple attribute handling

* added better comment

* made snapshot ignore invalid input as if it is invalid input in the original DOM, it should stay invalid in the recreated DOM

* added extra absolute test case

* code style

* addressed comments

test/snapshot.test.ts

@@ -5,7 +5,7 @@ import { absoluteToStylesheet } from '../src/snapshot';
 describe('absolute url to stylesheet', () => {
   const href = 'http://localhost/css/style.css';
 
-  it('cam handle relative path', () => {
+  it('can handle relative path', () => {
     expect(absoluteToStylesheet('url(a.jpg)', href)).to.equal(
       `url('http://localhost/css/a.jpg')`,
     );