Add utility function with tests to pad datetime digits and validate

[ibnesayeed]

Pad datetime to make a 14-digit string and optionally validate it as per #301.

[codecov]

Codecov Report

Merging #586 into master will increase coverage by 0.78%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage    23.6%   24.38%   +0.78%     
==========================================
  Files           6        6              
  Lines        1199     1218      +19     
  Branches      179      181       +2     
==========================================
+ Hits          283      297      +14     
- Misses        895      900       +5     
  Partials       21       21

Impacted Files	Coverage Δ
ipwb/replay.py	13.95% <0%> (-0.11%)	⬇️
ipwb/util.py	34.5% <100%> (+4.93%)	⬆️

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[machawk1]

Tested and LGTM!