Add support for integrity attribute in script/link

[qligier]

When the integrity attribute is used on script/link elements, it isn't updated after optimization of the JS/CSS content and prevents the browser from evaluating the resource.

E.g.

<link rel="stylesheet"
         href="/css/style.min.css"
         integrity="sha256-wa1ES7wqg7gaI5M1FpSm/2c4k9EifIgNRSlhJzJxlgw=" crossorigin="anonymous">

Resulting in:

Failed to find a valid digest in the 'integrity' attribute for resource '/css/style.min.css'
with computed SHA-256 integrity 'PuEGdAutN3Hy2YTBDNjBfmmg8LRELm2YW2VO/GqF6yI='.
The resource has been blocked.

It would be nice to have support for this attribute. Thank you!