Add load_lastfm data loader to get the user-artist listening frequency data

[takuti]

Relates to #29

[codecov-commenter]

Codecov Report

Merging #43 (2096051) into master (42fcd36) will decrease coverage by 21.03%.
The diff coverage is 3.33%.

@@             Coverage Diff             @@
##           master      #43       +/-   ##
===========================================
- Coverage   98.28%   77.24%   -21.04%     
===========================================
  Files          22       22               
  Lines         641      668       +27     
===========================================
- Hits          630      516      -114     
- Misses         11      152      +141     

Impacted Files	Coverage Δ
src/datasets.jl	11.32% <3.33%> (-88.68%)	⬇️

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[takuti]

Coverage failure is expected as we skip the test cases involving a download operation: 2096051