[FEATURE] Add support for strict mode feature

[gogaz]

Handlebars.js allows end-user to provide the option strict: true to their API (documented here)

In this mode, templates will throw rather than silently ignore missing fields. This has the side effect of disabling inverse operations such as {{^foo}}{{/foo}} unless fields are explicitly included in the source object.

This pull request adds support for the strict mode and has the same API as the original Handlebars library.