FR: Template tags syntax suggestion

Template tags like `{{concerning}}` are already great especially with the pipe symbol.

One enhancement could be to make a prefix or suffix dependent on whether the template has a value. For instance:

1. `{{_:correspondent}}` would expand to `_John` if the correspondent was set, else the underscore would not be rendered;
2. `{{correspondent|concerning:_}}` would expand to `John_` if …