Cache rendered HTML on element objects

Test: manipulation - html render (file: jquery.html)
  cheerio x 814,945 ops/sec ±3.04% (84 runs sampled)
  old x 208 ops/sec ±3.97% (79 runs sampled)
  jsdom x 51.31 ops/sec ±4.01% (56 runs sampled)

Test: manipulation - html independent (file: jquery.html)
  cheerio x 8,974 ops/sec ±5.33% (79 runs sampled)
  old x 8,787 ops/sec ±4.41% (85 runs sampled)
  jsdom x 252 ops/sec ±6.28% (70 runs sampled)