New feature: outline directives

Hi all,

I got bored of waiting for me to release TT3 so I back-ported "outline" 
directives to TT2.  This is only on github at the moment, not yet 
released to CPAN.

Short example:

   %% IF a
   a is set to [% a %]
   %% ELSE
   a is not set
   %% END

Which is equivalent to:

   [% IF a -%]
   a is set to [% a %]
   [% ELSE -%]
   a is not set
   [% END -%]

Enable it by setting the TAG_STYLE option to 'outline':

   my $tt = Template->new({
       TAG_STYLE => 'outline',
   });

Or by defining your own OUTLINE_TAG string:

   my $tt = Template->new({
       OUTLINE_TAG => '%%',
   });

Or by using the TAGS directive to set the 'outline' style:

   [% TAGS outline %]

Or to set the start, end and (optional) outline tags:

   [% TAGS <* *> >> %]

See t/outline.t for tests:

   https://github.com/abw/Template2/blob/master/t/outline.t

Changes to the relevant sections of the docs are here:

https://github.com/abw/Template2/commit/5622870ba1b7c6da3720c6b6cec29c0304ef3ce9#diff-6dd5714206375d47e8c26eb0fe6980a4R60

https://github.com/abw/Template2/commit/5622870ba1b7c6da3720c6b6cec29c0304ef3ce9#diff-65cc2456a43e5577bf3ac2a18eb43ebdR60

Comments, feedback, etc., welcome.  Assuming I don't hear any reports of 
mass breakage, I'll release this as version 2.26 in the next few days 
(noting that I've also got two pull requests outstanding on github that 
I need to look at first).

Cheers
Andy