introduce tags/value-or-var

[piranha]

It makes it's easier for tags to decide if they've received a string or a name of variable from context.

[danlarkin]

Nice change, but what do you think about having introduced a change to include (and extends) whereby now it can take not just a literal string, but a value from the template context?

[piranha]

Well, old behavior is preserved and new one added: before that you had to write {% include "template.html" %}, now you can use a variable...

It's not like I need include or extends to be dynamic (though they obviously can), it's just I needed such a function in my own template tag and thought it would be good to introduce it in library itself.