Add the -i/--in-place command-line option

[jhradilek]

When the -i/--in-place option is used, dita-convert overwrites the supplied files with an updated content instead of printing it to standard output.

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