Add a None option when pin is configured

[andrewdavidmackenzie]

Fixes #87

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 8%. Comparing base (61fe7cf) to head (3373720).

Files	Patch %	Lines
src/pin_layout.rs	0%	15 Missing ⚠️
src/piggui.rs	0%	6 Missing ⚠️
src/hw/none.rs	0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #110   +/-   ##
=====================================
- Coverage       9%     8%   -0%     
=====================================
  Files          10     10           
  Lines         908    914    +6     
=====================================
  Hits           73     73           
- Misses        835    841    +6     

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