Issue 104 advanced intermediate outputs

[csala]

Resolve #104

Allow defining lists of outputs in the pipeline JSON

Add new methods get_outputs, get_output_names and get_output_variables.
Modify output_ argument behavior in MLPipeline fit and predict methods.

[codecov-io]

Codecov Report

Merging #105 into master will increase coverage by 0.79%.
The diff coverage is 89.9%.

@@            Coverage Diff            @@
##           master    #105      +/-   ##
=========================================
+ Coverage   77.31%   78.1%   +0.79%     
=========================================
  Files           5       5              
  Lines         626     676      +50     
=========================================
+ Hits          484     528      +44     
- Misses        142     148       +6

Impacted Files	Coverage Δ
mlblocks/mlpipeline.py	87.72% <89.9%> (+2.26%)	⬆️
mlblocks/datasets.py	32.9% <0%> (-3.23%)	⬇️

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