add Subresource Integrity support

[hthetiot]

Subresource Integrity defines a mechanism by which user agents may verify that a fetched resource has been delivered without unexpected manipulation. In a nutshell, metadata inlined into HTML elements allows the browser to determine whether the resource that was downloaded matches the resource the page's author expected to download. This implementation covers only the two elements outlined in the spec: <script> and elements.

• https://www.chromestatus.com/feature/6183089948590080
• https://googlechrome.github.io/samples/subresource-integrity/