Correct the output file extension when reading from STDIN

[jhradilek]

When reading input from STDIN with the -d/--directory option specified, the dita-convert utility incorrectly names the output file out.adoc. This pull request changes the output file name to out.dita.

$ asciidoctor -r dita-topic -b dita-topic test.adoc -o - | dita-convert -gd out
$ ls out/
out.dita

Implementation checklist

• The code changes come with the corresponding test cases
• The code changes pass all tests (run python3 -m unittest in the project directory)
• The code changes come with appropriate documentation