support optional groups in templates

[mattfysh]

templates can currently interpolate optional variables, which typically results in the template (and object entry, if applicable) being evaluated to null and removed, for example:

GET http://example.com
User-Agent: $first__${second}123

set obj = {
  foo: value__$first__${second}123
}

if either first or second variables are null selections, the User-Agent header and object entry foo are dropped entirely.

in some cases, you might want optional variables to evaluate to an empty string instead of the whole template evaluating to null. there may also be pieces of the template that should only render if a variable is defined, e.g.GET http://example.com$[/page-$page] should evaluate to http://example.com when $page is null, and to http://example.com/page-3 when $page is 3