plugin and writer interfaces update

[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
• [ ] run 'make format' so as to confirm the pyexcel organisation's coding style
• [ ] Please add yourself to 'contributors' section of pyexcel-io.yml (if not found, please use CONTRIBUTORS.rst)
• [ ] Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #79 into dev will decrease coverage by 0.05%. The diff coverage is 86.36%.

@@            Coverage Diff             @@
##              dev      #79      +/-   ##
==========================================
- Coverage   96.93%   96.88%   -0.06%     
==========================================
  Files          51       51              
  Lines        3201     3212      +11     
==========================================
+ Hits         3103     3112       +9     
- Misses         98      100       +2     

Impacted Files	Coverage Δ
pyexcel_io/database/__init__.py	100.00% <ø> (ø)
pyexcel_io/database/importers/django.py	97.95% <ø> (-0.05%)	:arrow_down:
pyexcel_io/database/importers/sqlalchemy.py	95.00% <ø> (-0.09%)	:arrow_down:
pyexcel_io/plugins.py	96.29% <ø> (ø)
pyexcel_io/readers/__init__.py	100.00% <ø> (ø)
tests/test_book.py	100.00% <ø> (ø)
pyexcel_io/writers/csv_in_file.py	95.23% <85.71%> (-4.77%)	:arrow_down:
pyexcel_io/writers/csv_in_memory.py	95.00% <85.71%> (-5.00%)	:arrow_down:
pyexcel_io/writers/csvz_writer.py	95.83% <85.71%> (-4.17%)	:arrow_down:
pyexcel_io/writer.py	100.00% <100.00%> (+3.33%)	:arrow_up:

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