zshrc: add mkcd function
zsh: increment.zsh -> functions/increment-number

Using autoload is the standard way of writing ZLE modules, so I will follow
the standard rather than using my own solution.
zsh: use no_ksh_arrays in increment-number

Our script breaks if ksh_arrays is set, so unset it to be safe.
zsh: add cycle-quotes
update README with zsh features