Drop cache for particular arguments

[richardkunze]

This PR addresses #66.

I added a function drop_cache working exactly like has_cache but removing the cache for the given arguments. It should support all possible caching functions but I only tested memory and filesystem due to missing S3/GCS credentials.

[codecov-io]

Codecov Report

Merging #78 into master will decrease coverage by 16.68%.
The diff coverage is 69.23%.

@@             Coverage Diff             @@
##           master      #78       +/-   ##
===========================================
- Coverage   76.41%   59.72%   -16.69%     
===========================================
  Files           5        5               
  Lines         195      221       +26     
===========================================
- Hits          149      132       -17     
- Misses         46       89       +43

Impacted Files	Coverage Δ
R/cache_gcs.R	0% <0%> (ø)	⬆️
R/cache_s3.R	0% <0%> (-100%)	⬇️
R/cache_filesystem.R	100% <100%> (ø)	⬆️
R/memoise.R	100% <100%> (ø)	⬆️
R/cache_memory.R	100% <100%> (ø)	⬆️

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[jimhester]

This looks good, thanks! I think all that is missing is adding a note to NEWS.md detailing the change, with this PR number and your GitHub username.

[jimhester]

Thanks!