Add asdict method that returns JSON-encodable object

[lopuhin]

Explanation object can still contain numpy objects (it may be faster and more convenient), transformation to plain python objects happens in numpy_to_python object.

Transformation in asdict loses information anyway (at least it loses types of objects), but here another loss of information occurs for FormattedFeatureName where we just take it's value and can not distinguish formatted and not formatted feature names. I think that this might be better to make feature name a wrapper object in any case (we also have unhashed features represented as lists) - but then it's better to do it separately.

[codecov-io]

Current coverage is 96.43% (diff: 96.00%)

Merging #84 into master will decrease coverage by 0.01%

@@             master        #84   diff @@
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  Files            30         30          
  Lines          1407       1431    +24   
  Methods           0          0          
  Messages          0          0          
  Branches        258        268    +10   
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+ Hits           1357       1380    +23   
  Misses           27         27          
- Partials         23         24     +1   

Diff Coverage	File Path
••••••••• 95%	eli5/base_utils.py
•••••••••• 100%	eli5/base.py

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