Add support for MapMyGenome

[willgdjones]

This PR adds support for MapMyGenome data files.

[codecov-io]

Codecov Report

Merging #29 into develop will increase coverage by 0.87%.
The diff coverage is 81.81%.

@@             Coverage Diff             @@
##           develop      #29      +/-   ##
===========================================
+ Coverage    88.76%   89.63%   +0.87%     
===========================================
  Files            5        5              
  Lines         1041     1052      +11     
  Branches       194      196       +2     
===========================================
+ Hits           924      943      +19     
+ Misses          72       62      -10     
- Partials        45       47       +2

Impacted Files	Coverage Δ
src/snps/io.py	85.23% <81.81%> (-0.12%)	⬇️
src/snps/resources.py	93.92% <0%> (+4.04%)	⬆️

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[apriha]

Thanks again @willgdjones ! Merging PR.