zsh: Move zsh startup files to topic folder.
Create master setup script and utility library.

The first step in creating a kind of "installer" for my Unix
configuration. The master script will find and source any other "setup"
files located in the local directory tree.

The utility library is meant to implement common functionality in
a POSIX-compliant manner. So far, it provides a function that accepts
relative paths and creates absolute-path symlinks.
zsh: Create script for zsh setup.
setup: Fix typo in setup utility library.
hg: Create topic folder for Mercurial.
setup: Separate out lists of link sources.
readline: Create topic folder.
git: Create topic folder.
setup: Don't support custom install locations yet.

Some of the dotfiles assume that they are located in the user's home
directory. Let's not support installing to custom locations until we
sort that out.
zsh: Bring back the alternative ZDOTDIR.

This will make it simpler to add topic-based zsh customization. The zsh
startup files will source relevant files from appropriate locations
within ZDOTDIR, and anything that wants to add behavior will just have
to add their customizations to the proper directory.
zsh: Put Colemak ZLE tweaks into the Colemak topic folder.

Also augment .zshrc to pick up any customizations in
$ZDOTDIR/interactive.
tmux: Put tmux Colemak keybindings into Colemak topic folder.
readline: Move customizations to Colemak topic folder.

Everything in inputrc is for Colemak, anyway.
tmux: Move customizations into a topic folder.
tmux, zsh: Move relevant customizations into tmux module.
lynx: Move key remappings into Colemak module.
lynx: Move customizations into new topic folder.
tmux: Look in ~/.coleslaw for Colemak keybindings.

It'll probably be easier in the long run to have most Colemak-related
customizations be in a single directory.
zsh: Use "zsh_history" instead of ".zsh_history".

Now that history isn't stored in $HOME, there's no real reason keep it
hidden.
setup: Don't allow custom target.

I have no idea how this is going to work yet, so we'll just leave it out
for now.