Fixed file formats might contain an header line with column names and a hint
of the size of each column. While it might be a long shot that we can
acutally use that as a proper fixed-format specification, this patch
implements a guess mode that also outputs the parsed header.
In case when the parsing is wrong in some level of details, it might
actually be a good start to copy/paste from the command output and go from
there.
Fixes#958.
