List all possible configuration names in the config(7) man page

This could be done by refactoring the list of config settings in each of the other man pages as data objects, then including the subsets of the data objects in each other man page, and the complete list of data objects in the config(7) man page.

(This might help with [#678](https://github.com/GothenburgBitFactory/timewarrior/issues/678) and https://github.com/GothenburgBitFactory/ti.net/issues/977)