Allow passing fit and produce args as init params

[csala]

Resolve #96

Allow passing fit and predict arguments that will not change throughout the pipeline lifetime as init_params.

Also, in order to support and properly test this:

• allow passing the primitive annotation dictionary directly to the MLPipeline instead of the primitive name
• allow passing the primitive function (or class) directly within the annotation instead of passing the name to be imported.

[codecov-io]

Codecov Report

Merging #102 into master will increase coverage by 0.29%.
The diff coverage is 96%.

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   77.02%   77.31%   +0.29%     
==========================================
  Files           5        5              
  Lines         618      626       +8     
==========================================
+ Hits          476      484       +8     
  Misses        142      142

Impacted Files	Coverage Δ
mlblocks/mlpipeline.py	85.46% <100%> (+0.64%)	⬆️
mlblocks/mlblock.py	91.3% <90%> (-0.52%)	⬇️

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