feature: with_suffix to correspond to with_prefix

[Borgerr]

Stumbled upon while trying to wrangle some integration testing for a CLI tool, which expects a specific filetype. with_prefix adds a short string to the start of a file's name. We can alter this very cleanly by simply doing some plumbing for adding a short string to the end of the file's name, instead. Builder already has a suffix method which we can leverage for this.

[Stebalien]

Seems reasonable, I'm happy to review a patch.