Change split_single_document to work on STDIN & STDOUT

[jelmervdl]

Single document splitting just directly goes from Perl's STDIN to STDOUT now. In multidoc mode I locally override STDIN and STDOUT to point to variables.

I still buffer a single document in multidoc mode because I didn't see an easy way to stream base64 without re-implementing base64-encode in Perl. Piping it through base64 with open() would work, but that would entail forking for every document.

Fixes #30. Also fixes a warning printed by $text = $text.$words[$i] when dealing with an empty line in -k mode.