Add option for appending semicolons

[mgkurtz]

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This was asked for in an old comment. My use case was to have a line

is-at-least 5.9 || zstyle ':autocomplete:*' append-semicolon off

as a quick fix for #797 on old machines, before I wrote #818.

I do assume, this option comes with the risk, that the next time some semicolon suffix struggle arises, people just deactivate this feature and then do not notice, when you fix it. So, merge at your own discretion.