use ISheetWriter 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 #37 into dev will decrease coverage by 0.12%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##              dev      #37      +/-   ##
==========================================
- Coverage   98.54%   98.41%   -0.13%     
==========================================
  Files          13       13              
  Lines         688      695       +7     
==========================================
+ Hits          678      684       +6     
- Misses         10       11       +1     

Impacted Files	Coverage Δ
pyexcel_xls/xlsw.py	98.21% <92.85%> (-1.79%)	⬇️
pyexcel_xls/xlsr.py	99.25% <100.00%> (-0.01%)	⬇️

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