Various changes to prepare for version 0.1

[pazzo83]

See issue here: #5

Changes involve:

• separating out transformer code from main module file
• updating README
• separating out transformer tests from main test file

[codecov-commenter]

Codecov Report

Merging #6 (a245758) into dev (1057d6a) will not change coverage.
The diff coverage is 80.24%.

@@           Coverage Diff           @@
##              dev       #6   +/-   ##
=======================================
  Coverage   80.48%   80.48%           
=======================================
  Files           1        2    +1     
  Lines          82       82           
=======================================
  Hits           66       66           
  Misses         16       16           

Impacted Files	Coverage Δ
src/MLJText.jl	100.00% <ø> (+19.51%)	⬆️
src/tfidf_transformer.jl	80.24% <80.24%> (ø)

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[storopoli]

Looks good. Just added a small suggestion in the README

[ablaom]

Looks good to me.