Don't use private container

[siemensikkema]

See #26

I think I removed the necessity for a private container by clearing the flashes out in the middleware. Needs testing when I have time.

[codecov]

Codecov Report

Merging #29 into master will not change coverage.
The diff coverage is 0%.

@@         Coverage Diff          @@
##           master   #29   +/-   ##
====================================
  Coverage       0%    0%           
====================================
  Files           8     6    -2     
  Lines         107    84   -23     
====================================
+ Misses        107    84   -23

Impacted Files	Coverage Δ
Sources/Flash/Providers/FlashProvider.swift	0% <ø> (ø)	⬆️
Sources/Flash/Models/Flash.swift	0% <ø> (ø)	⬆️
Sources/Flash/Extensions/Container+Flash.swift	0% <0%> (ø)
Sources/Flash/Tags/FlashTag.swift	0% <0%> (ø)	⬆️
Sources/Flash/Middlewares/FlashMiddleware.swift	0% <0%> (ø)	⬆️
Sources/Flash/Extensions/Future+Flash.swift	0% <0%> (ø)	⬆️

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