Rework autoescape to be a simple boolean; escape the escape character

Rework autoescape to be a simple boolean; escape the escape character

Reworked the new "autoescape" feature introduced in
:ref:`change_2694` in 1.2.0b2 to be fully automatic; the escape
character now defaults to a forwards slash ``"/"`` and
is applied to percent, underscore, as well as the escape
character itself, for fully automatic escaping.  The
character can also be changed using the "escape" parameter.

Change-Id: I74894a2576983c0f6eb89480c9e5727f49fa9c25
Fixes: #2694