add if syntax

[JunNishimura]

• handle with if expression in lexer (add if syntax #18)
• add new token 'TRUE' (add if syntax #18)
• handle with if expressions in parser (add if syntax #18)
• handle with true object (add if syntax #18)
• handle with if token in lexer (add if syntax #18)
• handle with if expression in parser (add if syntax #18)
• handle with comparison tokens in lexer (add if syntax #18)
• add unit tests for comparison in parser (add if syntax #18)
• expand builtin functions (add if syntax #18)
• evaluate if expressions (add if syntax #18)