Add position property to facets to make it easy to change the order in which they are rendered

[loevgaard]

Fixes #56

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 34.58%. Comparing base (78d8d2b) to head (66d0028).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Form/Builder/CheckboxFilterFormBuilder.php	0.00%	1 Missing ⚠️
src/Form/Builder/ChoiceFilterFormBuilder.php	0.00%	1 Missing ⚠️
src/Form/Builder/RangeFilterFormBuilder.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #58      +/-   ##
============================================
- Coverage     34.59%   34.58%   -0.02%     
  Complexity      619      619              
============================================
  Files           119      119              
  Lines          2081     2085       +4     
============================================
+ Hits            720      721       +1     
- Misses         1361     1364       +3     

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