add Char column type

[dantownsend]

Related to #364

[codecov-commenter]

Codecov Report

Merging #368 (b50a6a0) into master (144faba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #368   +/-   ##
=======================================
  Coverage   90.59%   90.59%           
=======================================
  Files         127      127           
  Lines        6411     6412    +1     
=======================================
+ Hits         5808     5809    +1     
  Misses        603      603           

Impacted Files	Coverage Δ
piccolo/apps/schema/commands/generate.py	91.42% <ø> (ø)
piccolo/columns/column_types.py	92.91% <100.00%> (+0.01%)	⬆️

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