Add option for FWF reader and writer

The request to handle fixed with format pops up frequently.

Here are the major points of the functionality (this is an ideal state - not all has to be implemented from the start):

• we need reader and writer
• for both reader and writer there be two options (both are needed in different situations)
  • byte based width counting
  • character based width counting
• column width specification
  • both reader and writer should have an "autodetection" of fields width based on a separator passed
  • both reader and writer should have an option to be passed a sequence of integers that indicate consecutive field widths
  • both reader and writer should have an option to be passed a sequence of integers that indicate consecutive field widths (e.g. [1,2,3])
  • both reader and writer should have an option to be passed a sequence of ranges (e.g. [1:2, 5:10], overlapping ranges should error) that indicate field widths (this allows for skipping fields - writer should fill gaps with separator)
• writer should allow to specify alignment of contents within field
• reader should optionally strip separator when reading in strings (i.e. field " ab " should be parsed as "ab" or " ab " depending on this option)
• consideration should be given if we allow the last column to have a different width in different rows when we use autodetection or width specified as integers (again - maybe this should be an option - either accept or throw an error)

If something is not clear please comment.

Start with the issue body and separate the proposed fixed-width reader and writer behavior into a smaller initial scope. Resolve the open choices around byte versus character widths, width specification, alignment, stripping separators, and variable final-column widths before implementation. Done should be a clearly agreed subset with corresponding reader and writer behavior.