Use an option file instead of `CITRE_CMD` pseudo tag to store the ctags command line

[AmaiKinono]

For now, citre stores ctags command line in the tags file itself (using a CITRE_CMD pseudo tag), and parses it when updating the tags file. This:

• is very, very hacky,
• may violate the tags file format,
• doesn't play well with other tools that may try to regenerate the tags file using command line.

Instead, citre should generate and use an option file, which is supported by ctags. It could be <project-root>/.ctags.d/0.ctags, or go into citre-tags-file-global-cache-dir if the user don't want their project dir to be polluted.

The --options option and FILES/Default optlib path list in the manpage may be useful.