Ability to enforce `_escaped_fragment_=`

[gorhill]

Background: https://twitter.com/thatcks/status/958776519765225473

Announcement:
https://webmasters.googleblog.com/2009/10/proposal-for-making-ajax-crawlable.html

Deprecation announcement:
https://webmasters.googleblog.com/2015/10/deprecating-our-ajax-crawling-scheme.html

This is a trivial addition. Despite the deprecation, it's unclear whether this will still be used for long by web masters. Because it is trivial to support, I added a raw setting named enforceEscapedFragment (default to true, i.e. enabled by default) to enforce the still used _escaped_fragment_= directive. This raw setting could be removed at any point in the future if ever it is found that there is no longer any point to support it.

uMatrix will convert back and forth #! to _escaped_fragment_= depending on whether 1st-party scripts are blocked.

Test case: https://groups.google.com/forum/#!topic/golang-nuts/F5xFHTfwRnY