Support annotations for specifying fields

[rossmacarthur]

Resolves #99

[codecov-io]

Codecov Report

Merging #115 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #115    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files           7      7            
  Lines         837   1119   +282     
======================================
+ Hits          837   1119   +282

Impacted Files	Coverage Δ
src/serde/model.py	100% <100%> (ø)	⬆️
src/serde/fields.py	100% <0%> (ø)	⬆️

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