Send Flask signals for cache hits and misses

[alanhamlett]

Uses Flask Signals to solve #220.

Adds 4 new signals:

• flask_caching.cache_view_hit
• flask_caching.cache_view_miss
• flask_caching.cache_memoize_hit
• flask_caching.cache_memoize_miss

Signals for cache.get aren't needed because the library user knows the result.

[codecov-io]

Codecov Report

Merging #237 (e93be24) into master (bfe1c5d) will decrease coverage by 4.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
- Coverage   89.63%   85.36%   -4.27%     
==========================================
  Files          20       21       +1     
  Lines        2499     2638     +139     
==========================================
+ Hits         2240     2252      +12     
- Misses        259      386     +127     

Flag	Coverage Δ
unittests	85.36% <100.00%> (-4.27%)	⬇️

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Impacted Files	Coverage Δ
flask_caching/__init__.py	88.65% <100.00%> (+0.36%)	⬆️
flask_caching/contrib/googlecloudstoragecache.py	0.00% <0.00%> (ø)

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[sh4nks]

Hi, is this PR ready to be merged?