* todos.el: Improve handling of jumping to a category, in

* todos.el: Improve handling of jumping to a category, in
particular that of an archive.
(todos-category-completions): Add optional parameter to restrict
completions of archive categories to current archive.  Exclude
archive from completion files.
(todos-read-category): When called from an archive, restrict
category completions to the archive.  Fix mistaken use of all
completion files instead of just those in which the chosen
category occurs.
(todos-archive-mode-map): Add new key binding.
(todos-jump-to-category): Rename second optional parameter and
extend its use to restricting category completions to the current
archive.
(todos-jump-to-archive-category): New command.