Add option to input/output column units for fixed width tables #386

Extend the io.ascii.FixedWidth reader to include a keyword arg that will specify that there is a row of unit specifiers after the column name specifiers (or at the top of the header if there are no column names). This will apply for both reading and writing fixed width tables.

This allows for outputting a table to a file in a format like Table.pprint with show_units=True, and then reading back that table with no information loss.