Allow allowing/blocking hosts by IP range

[blakestoddard]

Give allowHosts and denyHosts the ability to allow/block based on a netblock. We're using this change to block the reserved IP ranges to prevent users from trying to proxy images from those ranges.

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[codecov]

Codecov Report

Merging #236 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   89.16%   89.28%   +0.11%     
==========================================
  Files           6        6              
  Lines         674      681       +7     
==========================================
+ Hits          601      608       +7     
  Misses         50       50              
  Partials       23       23              

Impacted Files	Coverage Δ
imageproxy.go	84.83% <100.00%> (+0.39%)	⬆️

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[blakestoddard]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[willnorris]

wow, this is great... very clean! Just a couple of minor comments.

[willnorris]

Looks great, thanks!