FEATURE: <% include %> inherits scope of parent template

NOTE: the unit tests for this will fail until #2137 or #2113 are merged since it uses a code branch that currently has a bug on 3.1 (and either of those pull requests will fix the bug).

This pull request represents a debated topic - whether or not the <% include %> tag should inherit the scope of the caller.  For the discussion, see https://groups.google.com/d/msg/silverstripe-dev/ymCg3m5LBxE/Ccn-8Gzk-3gJ

Since there is debate on this topic, the code is structured so that you can turn this feature on globally or on a per-include basis.  By default it is off - behaving exactly like 3.0 where scope is not inherited inside an include.

I am open to other suggestion on how this should be accomplished - for instance, I don't 100% like the "inheritScope=true" arg you can pass to an include.  But I wanted to get this prototype out here to aid with the discussion referenced above.