Make cachePathForKey public [resolves #255]

[jasonnoahchoi]

Understandably, the method was private; however, this commit solves the problem of anyone that requires access to the actual file path of the cached image. This is especially useful for projects that are building iOS apps that are showing content with UIWebViews and lacks access to Internet connection.

Ref: #255

[codecov-io]

Current coverage is 76.90%

Merging #256 into master will not affect coverage as of b0dcce2

@@            master   #256   diff @@
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  Files           11     11       
  Stmts         1260   1260       
  Branches         0      0       
  Methods          0      0       
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  Hit            969    969       
  Partial          0      0       
  Missed         291    291       

Review entire Coverage Diff as of b0dcce2

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[onevcat]

Thanks, it makes sense.