Add option to use hierarchical sections to allow more then 2 levels in a ConfigSource

[bwindsor]

Closes #5

[codecov-commenter]

Codecov Report

Merging #9 (da2758a) into master (de0e2f0) will decrease coverage by 0.98%.
The diff coverage is 91.42%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage   96.91%   95.92%   -0.99%     
==========================================
  Files           5        5              
  Lines         259      270      +11     
  Branches       45       46       +1     
==========================================
+ Hits          251      259       +8     
- Misses          5        7       +2     
- Partials        3        4       +1     

Impacted Files	Coverage Δ
typedconfig/config.py	94.23% <81.25%> (-1.77%)	⬇️
typedconfig/provider.py	100.00% <100.00%> (ø)
typedconfig/source.py	100.00% <100.00%> (ø)

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