Implement nesting

[kumarshantanu]

(Note: This is complex for the user. See #9 directives as an alternative.)

Implement nesting as follows:

placement	slot begin	slot end
nesting-begin	<%#	%>
nesting-contd	<%%	%>
nesting-end	<%/	%>

Notes:

1. Independent <%..%> slots are to be compiled as-it-is.
2. Matching nested slots (markers shown above) are to be compiled into a single block.
3. Nesting depth may be arbitrary as long as the template is well formed.

For the mustache template below

{{#items}}
  {{#first}}
    <li><strong>{{name}}</strong></li>
  {{/first}}
  {{#link}}
    <li><a href="{{url}}">{{name}}</a></li>
  {{/link}}
{{/items}}

the equivalent Basil template would look like as follows:

<%# (in items ; explodes each element in 'items', concats all form results
      (in first %>
        <li><strong><%name%></strong></li><%% )
      (in link %>
        <li><a href="<%url%>"><%name%></a></li><%/ )) %>