feat: Add support for variant

[martin-majlis]

This PR is introducing support for language variants. You can now construct client as:

wiki_zh_tw = wikipediaapi.Wikipedia(user_agent, "zh", "zh-tw")
zh_page_tw = wiki_zh_tw.page("Python")
print(zh_page_tw.title + ": " + zh_page_tw.fullurl)
print(repr(zh_page_tw.varianttitles))

and fetch content in taiwanese mandarin.

Fixes #209

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.99%. Comparing base (1534c79) to head (b8ca892).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
wikipediaapi/__init__.py	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   93.49%   93.99%   +0.50%     
==========================================
  Files           1        1              
  Lines         415      433      +18     
==========================================
+ Hits          388      407      +19     
+ Misses         27       26       -1     

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[XapaJIaMnu-at-meta]

Yes, Thank you! This is exactly what i wanted! It is true not all languages have variants (In fact, I think it's only Serbian and Chinese). Could you also add the example in the readme, pretty please (: