Renamed to 'files.lua' since it is the only module actively working on
the underlying filesystem so far -- gathering existing files and zettel
in the directories.
Options were previously set ad-hoc in the initialize function, but
re-set and overwritten at various places.
Options now live in one central module (options.lua), where they
directly access the neovim options set, and re-access them on any run.
That means options can be changed while the plugin is running and it
will use their newer versions, without requiring any re-initialization.
Moved list functionality into separate file list.lua.
Moved zettelkasten options from local variables to table-scoped
variables, initialized in ZK.init().
