Add Per option

[rabbbit]

Replaces #21

It seems a bunch of people were interested in the feature (see #21),
it makes sense and it's super easy to add.

We also closed #17 in favour of #21, so it's nice to bring it back.

[codecov]

Codecov Report

Merging #54 (3247f7b) into master (11b8621) will increase coverage by 1.74%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   63.33%   65.07%   +1.74%     
==========================================
  Files           3        3              
  Lines          60       63       +3     
==========================================
+ Hits           38       41       +3     
  Misses         22       22              

Impacted Files	Coverage Δ
limiter_mutexbased.go	0.00% <0.00%> (ø)
limiter_atomic.go	100.00% <100.00%> (ø)
ratelimit.go	100.00% <100.00%> (ø)

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[cinchurge]

lgtm!