don't use \midrule between data rows in booktabs mode

cf. yihui/knitr@90effdfaf6adadc73
In booktabs mode, add \addlinespace each fifth line

- the second-to-last and last lines are treated especially
test corner cases, too
Merge pull request #28 from krlmlr/666-kable-booktabs

In booktabs mode, add \addlinespace each fifth line