Improve reader interface

[chfw]

With your PR, here is a check list:

• [ ] Has test cases written?
• [ ] Has all code lines tested?
• [ ] Has make format been run?
• [ ] Please update CHANGELOG.yml(not CHANGELOG.rst)
• [ ] Passes all Travis CI builds
• [ ] Has fair amount of documentation if your change is complex
• [ ] Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #90 into dev will increase coverage by 0.16%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #90      +/-   ##
==========================================
+ Coverage   97.03%   97.19%   +0.16%     
==========================================
  Files          50       52       +2     
  Lines        3239     3282      +43     
==========================================
+ Hits         3143     3190      +47     
+ Misses         96       92       -4     

Impacted Files	Coverage Δ
pyexcel_io/_compact.py	70.83% <ø> (+7.87%)	:arrow_up:
pyexcel_io/writers/csvz_sheet.py	100.00% <ø> (ø)
pyexcel_io/database/exporters/django.py	100.00% <100.00%> (ø)
pyexcel_io/database/exporters/queryset.py	100.00% <100.00%> (ø)
pyexcel_io/database/exporters/sqlalchemy.py	100.00% <100.00%> (ø)
pyexcel_io/database/importers/django.py	100.00% <100.00%> (ø)
pyexcel_io/database/importers/sqlalchemy.py	96.29% <100.00%> (-0.07%)	:arrow_down:
pyexcel_io/database/querysets.py	95.45% <100.00%> (+0.33%)	:arrow_up:
pyexcel_io/plugin_api/__init__.py	100.00% <100.00%> (ø)
pyexcel_io/plugin_api/abstract_reader.py	100.00% <100.00%> (ø)
... and 17 more

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