Replace UnicodePlots dependency with custom `braille` implementation

[adrhill]

Closes #83.

• removes UnicodePlots from dependencies
• adds PaddedViews and TiledIteration dependencies
• adds braille kwarg to_string to return a string instead of printing to terminal

[codecov[bot]]

Codecov Report

Merging #91 (4c90d16) into master (81f26dd) will increase coverage by 0.25%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   97.69%   97.95%   +0.25%     
==========================================
  Files          12       12              
  Lines         260      293      +33     
==========================================
+ Hits          254      287      +33     
  Misses          6        6              

Impacted Files	Coverage Δ
src/api/default_method.jl	100.00% <ø> (ø)
src/braille.jl	100.00% <100.00%> (ø)
src/colorschemes.jl	100.00% <100.00%> (ø)
src/utils.jl	100.00% <100.00%> (ø)

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