--display-offset has had an odd relationship with negated inputs over its lifetime. Historical behaviors (and the addition being proposed here) might be easiest to understand in a tabular format.

This PR proposes to make negative values for --display-offset meaningful again by allowing them again when --skip is specified, adding a negative value to --skip if provided. To use the same format as above:

More detailed overview of the design and rationale:

• This stays consistent with current behavior -- the final display offset retains its concept as "--skip + --display-offset".

• This adds an error case for the display offset underflowing when added to the unsigned --skip offset:

  hexyl --display-offset=-21 --skip 20 .\some_file
  Error: invalid value when applying `--display-offset` to `--skip`
  
  Caused by:
      base (20) - offset (21) < 0
  

• Bonus: the error case where offset addition overflows now has a nice diagnostic:

  TODO
  

• If --skip is not specified, it is an error, as before:

  Error: failed to parse `--display-offset` arg "-1" as byte count
  
  Caused by:
      negative offset specified, but only positive offsets (counts) are accepted in this context
  

Resolves #112.

PR checklist:

• Get deets from PR OP into commit messages for history's sake.
• Tests