dumb-jump: Specify selector from completion framework used
dumb-jump: Specify selector from completion framework used

This commit adds a conditional which allows `dumb-jump` to use a selector
matching the current completion framework used (`ivy` or `helm`), instead
of the default popup. This relates to work done for adding Helm support
to `dumb-jump` here: https://github.com/jacktasia/dumb-jump/pull/96

The motivation behind this is two-fold: the fact that popups are known
to be broken when used with conflicting packages, such as `fci-mode` for
showing a column marker, and the need for better completions with the
possiblity for fuzzy-matching. This should hopefully lead to a more
consistent experience as well.

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs