Add header_padding option to pad headline fields

Example with header_padding=1 (and other PR for header width fix, overline + underline fix, custom THEME to make it more rst-friendly):

    =========  ========  =======  =======  =======  ======
     pcap ts    req ts    cmt      et       st       note
    =========  ========  =======  =======  =======  ======
    2.58       0         0.167
    5.51       2.93      3.07     3.07     0
    15.52      12.93     12.98    12.98    3.07
    25.51      22.93     22.923   22.923   12.98
    53.52      50.93     50.684   50.684   22.923
    93.52      90.93     90.157   90.157   50.684
    133.51     130.93    129.663  129.663  90.157
    161.97     159.39    157.71   157.71   129.663

It doesn't center fields, but makes them easier to read in case of such small names as above (imho, of course).