add `comments` argument to text readers

[andreas-h]

It would be very helpful if read_table and read_csv would support a comments keyarg, like np.loadtxt, to handle filesets with varying numbers of headers which are indicated by a comment character at the beginning.

[wesm]

I agree that would be nice. That doesn't seem to be what the comments option does, though, it actually splits comments off of any line, maybe have to dig a little deeper

[changhiskhan]

note that the comment keyword will not comment out a whole line and will only split comments off the end of a line