Sheet writer 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 #87 into dev will increase coverage by 0.00%. The diff coverage is 95.65%.

@@           Coverage Diff           @@
##              dev      #87   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
=======================================
  Files          50       50           
  Lines        3222     3225    +3     
=======================================
+ Hits         3126     3129    +3     
  Misses         96       96           

Impacted Files	Coverage Δ
pyexcel_io/database/importers/sqlalchemy.py	96.36% <85.71%> (+0.06%)	:arrow_up:
pyexcel_io/database/importers/django.py	100.00% <100.00%> (ø)
pyexcel_io/plugin_api/abstract_sheet.py	100.00% <100.00%> (ø)
pyexcel_io/writers/csv_sheet.py	100.00% <100.00%> (ø)
pyexcel_io/writers/csvz_sheet.py	100.00% <100.00%> (ø)

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