Implement relative line numbers.

There's an indented preference under "Show line numbers" labeled
"count lines relative to the cursor". Additionally, :set commands for
relativenumber, rnu, norelativenumber, and nornu work.

A notable difference from vim's implementation is that relative numbering is a
setting on top of numbering. So :set norelativenumber will *not* disable line
numbers altogether as it does in vim.