Chapter 34 grep operator plugin destroys the record of the most recent visual selection

Currently, the final implementation of the grep-operator plugin in chapter 34 destroys any record of the user's most recent visual selection when the operator is applied with a characterwise motion.

I have added a simple fix for this issue and updated some surrounding text in the "Saving Registers" section to explicitly mention the issue. This seemed an appropriate place to address the issue since the section is advocating that plugin authors "should *always* strive to save and restore any settings or registers [their] code modifies so [they] don't surprise and confuse [their] users."