Allow negative values for `--display-offset`

[ErichDonGubler]

--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.

When	Behavior summary	Negative offset behavior
First implemented in #72.	Absolute position, positive values only.	Disallowed.
Refined to allow negatives in #99.	Absolute position, including then-new semantics for negatives.	Allowed, end offset backwards from EOF (and so only usable on files), prompting #112 for poor diagnostics at the time.
Actual compatibility with xxd desired, made an offset of --skip in #119.	Relative offset based on --skip.	Disallowed.

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:

When	Behavior summary	Negative offset behavior
This PR.	Relative offset based on --skip.	Allowed when --skip is specified; underflow results in an error.

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.

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PR checklist:

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

[sharkdp]

Sounds great. Thank you very much for looking into this.

[sharkdp]

I'm closing this due to inactivity. Please feel free to comment in case it should be re-opened.