Add Excel CDF file detection

[FernandoDoming]

No description provided.

[coveralls]

Coverage increased (+0.05%) to 92.871% when pulling 7c9cc4c on FernandoDoming:excel_ole into 6009ffa on jbremer:master.

[jbremer]

Sweet, can you merge this functionality into the office_ole function though, so ole.listdir() is only invoked once?

[codecov-io]

Codecov Report

Merging #20 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #20      +/-   ##
=========================================
+ Coverage   93.29%   93.3%   +0.01%     
=========================================
  Files          23      23              
  Lines        1044    1046       +2     
=========================================
+ Hits          974     976       +2     
  Misses         70      70

Impacted Files	Coverage Δ
sflock/ident.py	88.57% <100%> (+0.33%)	⬆️

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[coveralls]

Coverage decreased (-0.3%) to 92.543% when pulling cccc64d on FernandoDoming:excel_ole into 6009ffa on jbremer:master.

[coveralls]

Coverage increased (+0.01%) to 92.83% when pulling cccc64d on FernandoDoming:excel_ole into 6009ffa on jbremer:master.

[jbremer]

All tests passed, excellent. Thanks!