Generic default value

[pgeiem]

Currently, when trying to access an inexistent value from a template the full rendering throw an exception.
In some case with large template it would be interesting to render the template even if there is any not found values.
The not found values could then be rendered as empty string or fixed constant.

Today to get this behaviour we have to explicitly use the default function for every single variable in the template.
This could be added as an option to Environment such as set_throw_at_missing_includes

Is there any possibilities to add this function ?