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.
Merge pull request #143 from vincentbernat/fix/yank-pop

zle: don't override yank/yank-pop