Change the diff background colours from their defaults to make the text
in the foreground easier to read.

I've been having some trouble reading comments, as the grey is rather
low contrast. To improve readability, change the highlight colour to a
lighter grey, increasing contrast.

After upgrading to Vim 8.2, an underline was added to the line number of
the current line. Remove this with `CursorLineNr` to restore the look
from before the upgrade.

Highlight background colour of the current line. Set it to a couple
stops lighter than the background colour.

Use a dark grey colour on statuslines of inactive windows to make it
easier to differentiate between active and inactive windows. The active
window remains the old light grey.

.vimrc from today. Will be re-adding my plugins as git submodules.