DOM diff view in CrawlOverview

Hi @crawljax/owners,

I needed an easy way to visualise the differences between state DOMs after crawling, and the CrawlOverview plugin seemed like a suitable place :)

I therefore added a tab to the "state" pages of the plugin, on which the diff with the previous state is shown. Users can also choose other files manually.

See [my website](http://iplantevin.net/ct/co1/states/index.html) for a simple demo crawl report with the new tab.

I thought I'd share this with you as a pull request on 4.0, see the commit for some more details.

Best regards,

Ivan