Adding number2word reverse function

[OptimusRhine]

• switched number system dicts to bidicts
• added num_to_word() function, which uses the same dict(s) to reverse the process of word_to_num()
• added pos + neg unit tests for new num_to_word() function
• updated readme
  • added usage instructions for num_to_word()
  • added personal contributor info

[codecov[bot]]

Codecov Report

Merging #35 into master will increase coverage by 0.7%. The diff coverage is 100%.

@@           Coverage Diff            @@
##           master     #35     +/-   ##
========================================
+ Coverage    96.8%   97.5%   +0.7%     
========================================
  Files           2       2             
  Lines         125     160     +35     
========================================
+ Hits          121     156     +35     
  Misses          4       4

Impacted Files	Coverage Δ
word2number/w2n.py	96.11% <100%> (+0.99%)	:arrow_up:
unit_testing.py	100% <100%> (ø)	:arrow_up:

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[akshaynagpal]

w2n has no plans to support reverse conversion for now. But, good effort.