Add support for Tables.jl interface

[malmaud]

Fixes #77.

[codecov-commenter]

Codecov Report

Merging #78 (ea69e66) into master (de05d58) will decrease coverage by 6.58%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
- Coverage   70.33%   63.75%   -6.59%     
==========================================
  Files           4        5       +1     
  Lines         236      240       +4     
==========================================
- Hits          166      153      -13     
- Misses         70       87      +17     

Flag	Coverage Δ
unittests	63.75% <90.00%> (-6.59%)	⬇️

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Impacted Files	Coverage Δ
src/tables.jl	83.33% <83.33%> (ø)
src/Pandas.jl	62.37% <100.00%> (-7.99%)	⬇️
src/util.jl	0.00% <0.00%> (ø)
src/tabletraits.jl	96.42% <0.00%> (+0.13%)	⬆️

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