zshrc: add pager_if_tty
vimrc: remove gitgutter sign colors, update line colors

The latest version of gitgutter automatically uses the right sign column
background color so we don't need to define it ourselves.  The line colors are
still too bright (those of diff) so we now use some very subtle line colors
that match our background color.

Here is the thread describing the bug referenced in the comment:
https://www.mail-archive.com/vim_use@googlegroups.com/msg45293.html
vim: fix gitgutter \gl mapping
vim: add \gh / \gi to swap gitgutter diffbase

\gh switches to diffing from HEAD, while \gi switches to diffing from the
index.
vim: make sign column much more subtle

The old color make it stand out, which I found distracting.  This new color is
only very subtly darker than the regular background color.