Add callback when no updates are available or previously ignored or marked for remind later

[ajilo297]

I am trying to add an additional callback for when no updates are available. This will be very handy for a few scenarios. For example, I would like to check if there are updates available when my app is launched. The best way to do this would be to do it in the Splash screen, and navigate to my inner pages if no updates are found.

Let me know your thoughts on this @larryaasen

[codecov[bot]]

Codecov Report

Merging #130 (5bb774d) into master (98ba530) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   90.60%   90.65%   +0.04%     
==========================================
  Files           9        9              
  Lines         777      781       +4     
==========================================
+ Hits          704      708       +4     
  Misses         73       73              

Impacted Files	Coverage Δ
lib/src/upgrade_alert.dart	100.00% <ø> (ø)
lib/src/upgrade_card.dart	100.00% <ø> (ø)
lib/src/upgrade_base.dart	66.66% <100.00%> (+1.44%)	:arrow_up:
lib/src/upgrader.dart	87.33% <100.00%> (+0.08%)	:arrow_up:

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