added cache-control header

[hellsan631]

This allows caching of images sent via download, and is very important to reduce bandwidth and for offline-capable web apps.

really loving the plugin btw :)

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This change is 

[codecov-io]

Current coverage is 86.88%

Merging #3 into master will increase coverage by +0.11% as of 20971eb

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  Files            1       1       
  Stmts          121     122     +1
  Branches        13      13       
  Methods         27      27       
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+ Hit            105     106     +1
  Partial          0       0       
  Missed          16      16       

Review entire Coverage Diff as of 20971eb

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[jdrouet]

@hellsan631 nice idea, but I think that it should be a parameter given to the component. I will introduce something like this next week ;)

[hellsan631]

I originally thought it weird that all of my api calls were caching except this one. For a better separation of concern, the caching should be given to express and your plugin should follow the filter/compression that's in place.