[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)
• New feature (non-breaking change which adds functionality)
• Maintenance change (non-breaking change such as upgrading a dependency, refactoring, or making a lint fix)
• 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

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

Additional Information

[codecov]

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

@@           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	Coverage Δ
lib/src/display.dart	100.00% <100.00%> (ø)
lib/src/jiffy.dart	98.81% <100.00%> (+0.02%)	⬆️

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