Have Vim set Terminal.app's proxy icon.

This is the same trick I use in zshrc and stole from OS X's /etc/bashrc.
I don't think I can send arbitrary escape sequences with Vim, so I abuse
its ability to set xterm titles.
Have Vim NOT set Terminal.app's proxy icon.

This reverts commit 9c0e52912dcc628eee57c5844adf47b3eddef556 "Have Vim
set Terminal.app's proxy icon".

Caught up in the novelty of setting the proxy icon, I forgot that the
icon is meant to represent the current working directory. Setting it to
a file breaks opening new tabs or windows to the CWD (having "New
windows open with" or "New tabs open with" set to "Same Working
Directory" in Terminal Preferences).
Change the chpwd function into a zsh precmd hook.

Identifying the current working directory to Terminal using the chpwd
special function leaves some annoying holes. For instance, any time that
the present working directory changes without an explicit "cd".

    % cd /foo       # $PWD is /foo, Terminal's cwd is /foo
    % (cd /bar)     # $PWD is /bar in the subshell, Terminal's cwd is /bar
    %               # $PWD is back to /foo, Terminal's cwd is still /bar

I only care about Terminal having the correct working directory while
sitting at an idle prompt, so the best time to update it is just before
every prompt, using a precmd hook. (Coming full circle, you might
notice.)
Move stty setting back to zshrc.

On second thought, stty should only be mucked with for interactive
shells, so it fits in better in zshrc.
Make update_terminal_cwd URL-encode the dir path.

By percent-encoding the entire directory path, it should be able to
handle all valid directory names, including those containing characters
that are not valid in URLs. (Terminal.app can handle many URLs
containing disallowed characters, but better safe than sorry.)
Add highlight-toggle entry to Colemak lesskey file.

Hitting <Esc> U on a QWERTY keyboard now correctly toggles the search
highlight.
Remove reference to compinstall from zshrc.

Don't think I've ever run compinstall. Plus, that line just duplicated
the default setting.