Add :border_tag option to Owl.Box.new/2

[zachallaun]

:border_tag can be a sequence/sequences that, if present, will be passed to Owl.Data.tag/2 and used to wrap the symbols that make up box borders. This allows you to stylize the border without having to tag the entire box and ensure that all contents are also tagged.

Note: I also edited one doctest in f8b1549 to better support editors that are configured to automatically strip trailing whitespace on save. If you'd prefer to leave it as-is, I'm happy to rebase this PR to remove that commit.

[fuelen]

@zachallaun I'd be happy to merge the PR, but please drop the commit related to newlines. I think editors should have disabled automatic stripping of trailing whitespaces. They should be integrated with mix formater instead :) It does the job well and doesn't strip whitespaces in strings.

[zachallaun]

Fair enough! Rebased & pushed.

[fuelen]

Thank you!