zle: don't override yank/yank-pop
zle: don't override yank/yank-pop

`yank-pop` relies on the fact that the last zle command is `yank` or
`yank-pop` to work correctly. Rewriting them prevents this check to work
correctly breaking `yank-pop`.

This fix just disallow overriding of those two zle commands. As a
side-effect, syntax highlighting will not happen when using.

This fixes #99.

joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,fly19890211/zsh-syntax-highlighting,stuha/wbr-zsh,codedecay/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,x-way/zsh-syntax-highlighting,g5pw/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,joshuarubin/dotfiles,m0vie/zsh-syntax-highlighting,jimmijj/zsh-syntax-highlighting,joshuarubin/dotfiles,psprint/zsh-syntax-highlighting,zsh-readable/zsh-readable,zsh-users/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,ryexley/zsh-syntax-highlighting,P71/zsh-syntax-highlighting