Enforce search behavior according to &foldopen

[milsen]

Resolve #102. Sneaking with set foldopen-=search will now handle closed folds like single targets and skip over multiple search results inside of them. With set foldopen+=search, i.e. the default behavior, closed folds are opened when sneaking to them. Streak-mode behavior is not influenced by this patch.

[justinmk]

Thanks, I will test this later. Functional tests would help too, though it's fine if you don't want to.