Merge pull request #1 from mattly/adjust-highlights-for-coding

Adjust terminal highlighting colors for code.
updated see also
updated with mattly's iterm scheme
misc fixes
Subtle colors for line numbers
Merge pull request #2 from tamagokun/patch-1

Subtle colors for line numbers
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.
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.