Redesign Text datasets

[CarloLucibello]

part of #73

[codecov-commenter]

Codecov Report

Merging #113 (41d7d82) into master (24a5611) will increase coverage by 0.77%.
The diff coverage is 44.91%.

@@            Coverage Diff             @@
##           master     #113      +/-   ##
==========================================
+ Coverage   38.32%   39.10%   +0.77%     
==========================================
  Files          35       35              
  Lines        1208     1248      +40     
==========================================
+ Hits          463      488      +25     
- Misses        745      760      +15     

Impacted Files	Coverage Δ
src/datasets_text/ptblm.jl	6.25% <6.25%> (ø)
src/datasets_text/smsspamcollection.jl	51.85% <51.85%> (ø)
src/datasets_text/udenglish.jl	56.86% <56.86%> (ø)
src/MLDatasets.jl	100.00% <100.00%> (ø)
src/abstract_datasets.jl	25.00% <100.00%> (+10.71%)	⬆️

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