Ignore undofiles directory; add TODO to create it

This directory was specified in the previous commit, 791d22ae.

I also added a TODO to create that directory if it doesn’t exist. Right now, undofile functionality will be silently disabled if I use this .vim folder on another computer.
Create undofiles directory using .keep file

At first I tried to use Vim code to create the directory if it didn’t exist. But it’s much simpler to just use Git. Thankfully, a file named “.keep” won’t get in the way (unless, I suppose, I edit a .keep at the root of my drive – but I don’t expect to).
Reorganize “easy variable rename” mapping section

To make comments more accurate. Before, some comments only applied to the commented-out version.

This also makes that mapping easier to copy for other people – they know they can ignore the following comments, which are either specific to the comment-out version or notes to myself.
Change from solarized dark to light by default

The solarized dark color scheme is usually too low-contrast to read with my monitor settings, which are optimized for making dark-on-light text readable. I still want to keep those monitor settings because most dark-on-light text is unavoidable (web pages).