Allow this package to help with populating the integrity attribute

[advdv]

Thank you for this open source project, nice and focussed! Though, I would argue that package is in the perfect position to help developers to populate the "integrity" attribute: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity. But currently it doesn't seem possible to get access to the raw hash to format a string in the way the integrity attribute expects.

If there is interest in this I can contribute an implementation for this, the following approaches would be possible

1. Create a method fs.Integrity(name) that simply returns a value for the integrity attribute. It might be too closely tied to this specific use case
2. Create a method that allows arbitrary formatting something like Sprintf(name, format, ...args), "format" argument could be a string with formatting directives and args would always include the hash. But ordering could be awkward, and this seems to magical
3. Simply have a method that returns the raw hash: fs.HashOf(name), the caller can use the hash in whatever way they like.

Let me know what you think