Added a goodie: Nestable capture blocks (like pseudo-templates
inside your templates)
This template:
```
<% auto outer = capture(() { %>
Outer, first
<% auto inner = capture(() { %>
Inner, first
<% }); %>
Outer, second
<%= inner %>
<% }); %>
<%= outer %>
```
Evaluates to this:
```
Outer, first
Outer, second
Inner, first
```
a-la Rail's capture helper. It was slightly strange to implement,
and means tighter coupling between the template function and a
template context, but I think at this point that's alright, given
the two are yin and yang anyways.
Right now they don't lazily evaluate; would it be desirable for
them to do so?
