Add the -s/--split-topic command-line option

[jhradilek]

When the -s/--split-topic option is used, instead of converting individual topics as usual the dita-convert splits the supplied DITA topic into smaller topics for each section. If the sections have the outputclass attribute defined, it correctly converts them to specialized concepts, tasks, and references.

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