Options defined with taxy-magit-section-define-column-definer don't specify a group

[tarsius]

This results in warnings such as these:

Compiling /home/jonas/.config/emacs/lib/hyperdrive/hyperdrive-mirror.el...

In toplevel form:
lib/hyperdrive/hyperdrive-mirror.el:92:2: Warning: in defcustom for ‘hyperdrive-mirror-level-indent’: \
fails to specify containing group
lib/hyperdrive/hyperdrive-mirror.el:92:2: Warning: in defcustom for ‘hyperdrive-mirror-item-indent’: \
fails to specify containing group

(Also, maybe any space in PREFIX should be replaced with a dash?)

[alphapapa]

Oops, thanks for reminding me of this. Will take care of it.

Hm, it might be best for the user to be sure to specify the group in the file before these forms, so maybe in this library I need to document that. But there are a few other small changes/fixes I want to make to this functionality, too, so...