New style writer

[chfw]

reference: pyexcel/pyexcel-io#80

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-xlsxw.yml (if not found, please use CONTRIBUTORS.rst)
• Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #7 into dev will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev       #7   +/-   ##
=======================================
  Coverage   98.15%   98.16%           
=======================================
  Files           8        8           
  Lines         380      381    +1     
=======================================
+ Hits          373      374    +1     
  Misses          7        7           

Impacted Files	Coverage Δ
pyexcel_xlsxw/__init__.py	100.00% <100.00%> (ø)
pyexcel_xlsxw/xlsxw.py	100.00% <100.00%> (ø)
tests/base.py	98.18% <100.00%> (ø)
tests/test_bug_fixes.py	100.00% <100.00%> (ø)
tests/test_formatters.py	100.00% <100.00%> (ø)
tests/test_multiple_sheets.py	97.12% <100.00%> (ø)
tests/test_stringio.py	100.00% <100.00%> (ø)
tests/test_writer.py	97.14% <100.00%> (-0.08%)	⬇️

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