Add filter to allow opt-out of WebDAV Edit Document

When editing a document in Chrome, without support for WebDAV enabled on the server and without authorizing any plugins to load when in the document admin, I get a "Plug-in Blocked" error every time I load the page.

![screen shot 2014-11-12 at 2 29 06 pm](https://cloud.githubusercontent.com/assets/286171/5019901/4c9b39fc-6a78-11e4-8343-a20131248513.png)

WebDAV seems really cool, but I don't have the bandwidth to figure out how to serve it properly and securely in our current configuration. I think at least parts of this should be opt-in, as not everyone will be WebDAV savvy.

Adding a filter for `document_revisions_enable_webdav` to opt out seems like a straight forward approach. It at least avoids loading the markup, though the JS continues to be available. Happy to discuss others as well.

Great plugin, thanks! :)