Adds support for Bulgarian language

[aquilax]

Adds support for Bulgarian language. According to the official rules for basic transliteration: https://slovored.com/transliteration/rules.html

[codecov[bot]]

Codecov Report

Merging #66 (4088c6d) into master (f347b0c) will not change coverage. The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #66   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          135       141    +6     
=========================================
+ Hits           135       141    +6     

Impacted Files	Coverage Δ
languages_substitution.go	100.00% <100.00%> (ø)
slug.go	100.00% <100.00%> (ø)

[matrixik]

/rebase