Add filter for relation

[LIQRGV]

This should solve issue #2

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master       #15   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        55        60    +5     
===========================================
  Files              5         5           
  Lines            150       164   +14     
===========================================
+ Hits             150       164   +14     

Impacted Files	Coverage Δ	Complexity Δ
src/Struct/FilterStruct.php	100.00% <100.00%> (ø)	13.00 <7.00> (+5.00)

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