Ignore comment lines in read_csv parsing

This is the first part of #7470.
closes #2685 
Shouldn't you also test it seperate? Because by providing skiprows, you don't test if `header=1` is skipping the commented lines automatically (only testint that skiprows does remove also the commented lines)? So for example also  ``read_csv(StringIO(data), comment='#', header=1)`` should yield the same result?