(woman-topic-at-point-default): Renamed to woman-use-topic-at-point-default.

(woman-topic-at-point-default): Renamed to woman-use-topic-at-point-default.
(woman-topic-at-point): Renamed to woman-use-topic-at-point.
(woman-file-name): Reflect renames above.  Automatically use the
word at point as topic if woman-use-topic-at-point is non-nil.
Otherwise offer it as default but don't insert it in the
minibuffer.  Also use `test-completion' instead of `assoc' as
suggested by Stefan Monnier.