[ENHANCEMENT] Add an option for extra lines of search context to `nb search`

[Beethoven-n]

Most grep utilities (including git grep) have a -C option to print multiple lines of context for each matched line.
Interestingly, it seems that nb handles pretty much everything related to the actual display of the matched line on its own, so it doesn't seem quite feasible to use --utility or $NB_SEARCH_TOOL to display the extra context.
Maybe there could be an $NB_GREP_ARGUMENTS variable to use around line 17793?