argstr templating using modern string formatting (i.e. incl. attributes)

[tclose]

This will allow us to drop the somewhat clunky keep_extension attribute (it will still be the default behaviour if the template arg is a Path). If you don't want the extension you can just access the Path attributes directly (e.g. in_file.name, in_file.parent, etc...)

[effigies]

I vote for 1.0, as changing this will impact how tasks are written.

[tclose]

Ensure this addresses #418