Implements #1173 (closed).
Not sure if I did the settings part as it should be done; I just went through the thousand places referencing show-line-numbers and did the same for relative line numbers.
In actual vim when relativenumber is enabled, but number is disabled, you get relative line numbers without the current line number. In this PR I omitted that to make things simpler: show-line-numbers fully controls whether line numbers are shown, and show-relative-line-numbers just changes all non-current line numbers to be relative.