Add support for revving src attribute of video and source tags

This adds support so usemin can replace filerev references for the src attribute of video and source tags.

For example,
```<video src="video.webm”>``` becomes ```<video src=“video.1234.webm”>``` if the mapping is present from grunt-filerev.

Needed this for our frontend build @bonobos. Just signed the CLA so that might take some time.