gitignore: Sort with C ordering
gitignore: Ignore more dotfiles

None of these are config files.
bashrc: Dedupe code for PROMPT_COMMAND

Refactor common code out of kw_bg_light() and kw_bg_dark(). Now, those
functions simply set $kw_ps1_bg_color.
bashrc: Rename kw_bg_dark/light -> kw_ps1_dark/light

The names are now more accurate. The functions set the color of PS1.
bashrc: Fix PROMPT_COMMAND for Mac and Gnome

When opening new terminal tabs and windows I want to preserve the pwd.
This patch fixes that behavior on Mac OS and some versions of
gnome-terminal.

Each OS, and different releases of gnome-terminal, define PROMPT_COMMAND
to a different default value. Stop trying to guess what that default
value is. Instead, append my custom PROMPT_COMMAND to the default value.
README: Update

Remove text about installation. This git repo is meant to be used
in-place at `~/.git`.