Allow blocking elements by selector (#50)

* Extract method (isElementBlocked) and add tests

* Add blockSelector argument to snapshot

If blockSelector is passed, it will be matched against the element.

Reasoning: Mutating class names can get messy, so providing another hook
helps keep code clean by using data-attributes instead.

test/snapshot.test.ts

@@ -1,6 +1,7 @@
 import 'mocha';
+import { JSDOM } from 'jsdom';
 import { expect } from 'chai';
-import { absoluteToStylesheet } from '../src/snapshot';
+import { absoluteToStylesheet, _isBlockedElement } from '../src/snapshot';
 
 describe('absolute url to stylesheet', () => {
   const href = 'http://localhost/css/style.css';
@@ -83,3 +84,27 @@ describe('absolute url to stylesheet', () => {
     expect(absoluteToStylesheet(`url('')`, href)).to.equal(`url('')`);
   });
 });
+
+describe('isBlockedElement()', () => {
+  const subject = (html: string, opt: any = {}) =>
+    _isBlockedElement(render(html), 'rr-block', opt.blockSelector)
+
+  const render = (html: string): HTMLElement =>
+    JSDOM.fragment(html).querySelector('div')!
+
+  it('can handle empty elements', () => {
+    expect(subject('<div />')).to.equal(false)
+  })
+
+  it('blocks prohibited className', () => {
+    expect(subject('<div class="foo rr-block bar" />')).to.equal(true)
+  })
+
+  it('does not block random data selector', () => {
+    expect(subject('<div data-rr-block />')).to.equal(false)
+  })
+
+  it('blocks blocked selector', () => {
+    expect(subject('<div data-rr-block />', { blockSelector: '[data-rr-block]' })).to.equal(true)
+  })
+})