zsh: extend FPATH to ~/.zsh/functions; autoload every function in that directory

Signed-off-by: Ben Walton <bdwalton@gmail.com>
zsh: add tmuxmacs function

Signed-off-by: Ben Walton <bdwalton@gmail.com>
zsh: set ALTERNATE_EDITOR to provide a fallback for emacsclient

Signed-off-by: Ben Walton <bdwalton@gmail.com>
zsh: alias emacs to tmuxmacs

Signed-off-by: Ben Walton <bdwalton@gmail.com>