Add mutually exclusive and required groups

[carlobaldassi]

Fix #25
Fix #58

[codecov-io]

Codecov Report

Merging #89 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   96.07%   96.14%   +0.06%     
==========================================
  Files           4        4              
  Lines        1122     1140      +18     
==========================================
+ Hits         1078     1096      +18     
  Misses         44       44

Impacted Files	Coverage Δ
src/settings.jl	96.11% <100%> (ø)	⬆️
src/parsing.jl	96.82% <100%> (+0.1%)	⬆️

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