Provide an option to show closed boards, false by default

[benmatselby]

No description provided.

[codecov]

Codecov Report

Merging #1 into master will increase coverage by 2.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   55.35%   57.37%   +2.01%     
==========================================
  Files           3        3              
  Lines          56       61       +5     
==========================================
+ Hits           31       35       +4     
- Misses         24       25       +1     
  Partials        1        1

Impacted Files	Coverage Δ
cmd/trello/board/list.go	72.72% <75%> (+2.13%)	⬆️

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