Add support for stale-if-error Cache-Control directive

[westonruter]

Useful for Squid and other proxies to allow stale content to be served if the server has an error:

The stale-if-error HTTP Cache-Control extension improves availability
of some kinds of cached content by allowing servers and clients to
instruct caches to use stale responses when certain error conditions
are encountered.

See The stale-if-error HTTP Cache-Control Extension.

Also updates the plugin codebase to v1.2 from the WordPress plugin SVN repo.

[nikolay]

👍

[vnsavage]

Thanks for the PR. Do you think you can recreate it with the latest version of Batcache?

[westonruter]

See #58