Generalize into a modular configuration.

I'll soon be moving other Colemak-related configuration files into this
repository, so I'm modularizing it. Each module / subdirectory will
contain customizations for a particular piece of software.

The Vim bits go into their own module, too.
Add less, lynx, readline, tmux, and zsh.

Copied directly over from my dotfiles repository*. Check that for change
history, if necessary.

* https://github.com/larryv/dotfiles