Command line options for additional settings

[mikegilchrist]

It would be useful to allow some options to be set via the command line and/or a user ~/.config/text2qti file.

For example, the grade posting option (post_manually) is hard coded as false in ./text2qti/xml_assessment_meta.py. It would be helpful to have this as a `--post-manually' cli option instead.

Similarly, the feedback is solution: true must be added or removed to the .txt file to change its behavior. Ideally, we could have a --feedback-is-solution cli option.

It seems like @darencard 's pull request provides an example one could follow on an option by option basis (something I could do when I get some time). Ideally we'd be able to use the terminology already present in the code to create new options in a more general manner (not something I understand how to do).

label: enhancement