Get language from HTML

[satyamtg]

This fixes #35 by implementing the following two functions in zimscraperlib.zim.rewriting -

• find_language_in() - Finds the language using the order mentioned in #35
• find_language_in_file() - Finds the language using find_language_in() from a file

Tests are also implemented for the same.

[codecov[bot]]

Codecov Report

Merging #37 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #37   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines          756       777   +21     
=========================================
+ Hits           756       777   +21     

Impacted Files	Coverage Δ
src/zimscraperlib/zim/rewriting.py	100.00% <100.00%> (ø)

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