Optionally warn if template requires field missing from context

[scmorrison]

This modification will allow for printing warning messages when a field referenced in a template is missing from context.

Template::Mustache.render(
    '{{missing_field1}} {{missing_field2}} {{missing.field}}\n',
{}, :warn);

# Field not found ❮missing_field1❯
# Field not found ❮missing_field2❯
# Field not found ❮missing.field❯

render will continue with the current behavior without the :warn flag.

[softmoth]

Thanks for this patch! I can see this coming in handy during development....

[scmorrison]

Thanks!