Add high-level client code

[rahulc29]

The high-level client code would perform the following tasks

• factory methods to produce default implementations of the Lexer, Parser, and Generator APIs
• determine the mode of input (standard input, file stream, HTTP, etc) and constructs the appropriate lexer
• take ownership of the AST produced by Parser API
• call the generator functions for the AST
• provide the context for generation stage