[Feature] Add a FacetsManager

[titouanmathis]

Added

• Add a FacetsManager to easily filter content with pre_get_posts (#32)
• Add a facets_get('key') Twig function to easily get the value of a facets filter from the query string (#32)

Fixed

• Fix dependency conflict (#32)

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 64.97%. Comparing base (b7cb597) to head (5784a09).

Files	Patch %	Lines
src/Managers/FacetsManager.php	94.73%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #32      +/-   ##
============================================
+ Coverage     62.81%   64.97%   +2.15%     
- Complexity      194      209      +15     
============================================
  Files            14       15       +1     
  Lines           554      591      +37     
============================================
+ Hits            348      384      +36     
- Misses          206      207       +1     

Flag	Coverage Δ
unittests	64.97% <97.29%> (+2.15%)	⬆️

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