[ISSUE-163] Option to omit prefix and suffix from relative datetime functions

[jama5262]

What does this PR do?

Option to omit prefix and suffix from relative datetime functions

Issue Reference

Fixes #163

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Maintenance change (non-breaking change such as upgrading a dependency, refactoring, or making a lint fix)
• [x] Documentation update
• [ ] Breaking change (a fix or feature that would cause existing functionality to change)

Screenshots

If applicable, add screenshots to help explain your problem.

Checklist

• [x] Wrote additional tests, if needed
• [x] All tests have passed
• [x] I have updated the documentation accordingly, if needed.

Additional Information

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (c907281) 96.30% compared to head (7f052b4) 96.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #218 +/- ## ======================================= Coverage 96.30% 96.31% ======================================= Files 34 34 Lines 1544 1548 +4 ======================================= + Hits 1487 1491 +4 Misses 57 57 ``` | [Impacted Files](https://codecov.io/gh/jama5262/jiffy/pull/218?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jama+Mohamed) | Coverage Δ | | |---|---|---| | [lib/src/display.dart](https://codecov.io/gh/jama5262/jiffy/pull/218?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jama+Mohamed#diff-bGliL3NyYy9kaXNwbGF5LmRhcnQ=) | `100.00% <100.00%> (ø)` | | | [lib/src/jiffy.dart](https://codecov.io/gh/jama5262/jiffy/pull/218?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jama+Mohamed#diff-bGliL3NyYy9qaWZmeS5kYXJ0) | `98.81% <100.00%> (+0.02%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jama+Mohamed). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jama+Mohamed)

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