Convert Lexer to ES6 class

[TheOpenDevProject]

As I was doing some work in an ES6 environment and needed to use this Lexer I decided to convert the Lexer to an ES6 Class.

Lexer can still be called using Lexer.lex(str,options) via a static method.

Some aditional work is required in terms of including the rest of the libraries removed by the require eg using Webpack.ProvidePlugin() although this extra step could be removed with more work.

[codecov-io]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff          @@
##           master   #82   +/-   ##
====================================
  Coverage      96%   96%           
====================================
  Files           1     1           
  Lines         675   675           
====================================
  Hits          648   648           
  Misses         27    27

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