zsh: enable substring history search

For some reason the following is now missing in the
`zsh-history-substring-search` package:

    zmodload zsh/terminfo

    bindkey "$terminfo[kcuu1]" history-substring-search-up
    bindkey "$terminfo[kcud1]" history-substring-search-down

    # bind UP and DOWN arrow keys
    bindkey '^[[A' history-substring-search-up
    bindkey '^[[B' history-substring-search-down

This causes the arrow keys behave as normal, i.e. the package is
disabled.