Add support for Enums

[alysivji]

Closes #7.

Building off the work in #24 which added support for IntEnum, this PR adds support for regular Enums.

• update README with types accepted

[codecov-io]

Codecov Report

Merging #25 (412e666) into master (38c7e06) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
+ Coverage   96.10%   96.20%   +0.09%     
==========================================
  Files           7        7              
  Lines         154      158       +4     
==========================================
+ Hits          148      152       +4     
  Misses          6        6              

Impacted Files	Coverage Δ
finite_state_machine/state_machine.py	100.00% <100.00%> (ø)

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