Added th_attrs attribute which works similary to td_attr and tr_attr but
for TH elements. Changes the cols attribute to allow mixed values; a
numeric still specifies the number of columns to render but now an array
of values can be used to specify TH column values. The number of columns
is determine from the size of the array. Further, a comma-separated
string of column names can be used which is internally coverted to an
array and used as if it was specified as a normal array.
Thanks for lynlyn for the feature request.
