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

- kable_latex output also works booktabs mode with omitted header
two new parameters to kable_latex: midrule and linesep

- By default, in booktabs mode, an \addlinespace is added every fifth line
- The last two lines are treated especially: In a table with 6, 11, 16, ... lines, no space is added before the last line
- Corner cases with 0 and 1 rows are tested by the examples
- Remove two bogus tab characters