Support negative sections

[maghoff]

Imagined syntax: {{^field}}...{{/field}}

Standard usecase: Render a list, but show something different when that list is empty.

Other imagined usecase: An Option can be iterated over for the Some(_) case, it makes sense to have this syntax work for the None case. Similarly for Result.

When using a Result in a negative section, maybe expose the Err(_) value as {{.}}?

[maghoff]

Maybe also support booleans with {{^bool?}}