Added Dutch language

[cast42]

Hi,
i used your python script to count frequencies from the subtitles in Dutch language.
Next, I cleaned the obtained dictionary using the list of word from OpenTaal. The list contains correct spelled words in Dutch but no frequencies.
I manually reviewed all words that were not in the OpenTaal list and added the correct spelling and it's variants.

I hope this PR helps to add Dutch ('nl') as a language to the package pyspellchecker.

If there's anything I can do to further help, please let me know.

Thank you for making and sharing pyspellchecker!

Kind regards,
Lode

[codecov-commenter]

Codecov Report

Merging #162 (1847ea0) into master (282eb15) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #162   +/-   ##
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  Coverage   92.20%   92.20%           
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  Files           4        4           
  Lines         295      295           
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  Hits          272      272           
  Misses         23       23           

Files	Coverage Δ
spellchecker/spellchecker.py	99.11% <100.00%> (ø)