Unify predict and ranking for simplicity

[takuti]

since ranking is used as a simple wrapper of predict.

[codecov-commenter]

Codecov Report

Merging #51 (16cfe67) into master (37cc748) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   78.63%   78.40%   -0.24%     
==========================================
  Files          23       23              
  Lines         716      713       -3     
==========================================
- Hits          563      559       -4     
- Misses        153      154       +1     

Impacted Files	Coverage Δ
src/base_recommender.jl	96.00% <100.00%> (-4.00%)	⬇️
src/baseline/co_occurrence.jl	100.00% <100.00%> (ø)
src/baseline/most_popular.jl	100.00% <100.00%> (ø)
src/baseline/threshold_percentage.jl	100.00% <100.00%> (ø)

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