Improve StatusLine highlights for multiple windows

I use a lot of windows when I code, and the StatusLine highlights are crucial
for helping me keep track of where things are and where I am in the window.
Maybe blue isn't the best choice for the active bar, but I kind of like it so
far.
Adjust terminal highlighting colors for code.

Error         This wasn't standing out enough, emboldened.

TODO          Was showing up as white on light grey, now on red.

ColorColumn   The `ctermbg` was simply set to the regular background and the
              foreground highlighting was a middle-grey.  This produced
              odd-looking off-color characters that seemed like some sort of
              error on the occasional ragged line.  Adjusted `ctermbg` to be
              similar to the guibg, and disabled the foreground highlighting.
              If you want to disable this, you can do so with `set colorcolumn=0`

CursorColumn  Was set to no highlighting.  This is a useful feature for
              whitespace-sensitive languages like Python or CoffeeScript, and if
              you want to disable it you can do so with `set nocursorcolumn`

CursorLine    Was set to no highlighting.  This is a useful feature for code in
              general.  If you want to disable it you can do so with
              `set nocursorline`
make StatusLineNC colors similar to gui
Migrate base syntax groups to color variables

This covers all of the stuff under `:help w18`.  The UI chrome parts will be
harder, as there's a lot more one-offs and this function-calling style makes
those a bit more difficult.

The colors for the Nomral group will now toggle white/black based on the value
of &background.  Note that for `&background == 'dark'` it will use regular white
on a darker black background and when light will use lighter black on regular
white.

The GUI colors for `Constant` and linked groups were varying shades of
darkgreen, instead of the yellow they were grouped under, while the cterm values
were still darkyellow.  I have set these to the shade of dark yellow I chose for
the iTerm version of pencil, for the sake of consistency.  Changing these would
be easy, so long as they are consistent.