Rework CSV and Fixed files source fields options, see #116.

It's not possible to use a comma separator when using more than one
source field option at the same time, and for better readability the
options are to be found enclosed in squared brackets.

Also, it's now possible to spell out "from" and "for" keywords on the
source definitions, making it easier to read and maintain the load file,
as in this full example:

          (
           a from  0 for 10,
           b from 10 for  8,
           c from 18 for  8,
           d from 26 for 17 [null if blanks, trim right whitespace]
          )