Feature Request: add support for simple operations on user inputs

[anroopak]

Add support to perform simple operations on the user inputs

• User inputs may be printed in response using {{path.user_id}} where path is a system defined variable and contains values passed on the URL path and user_id is the variable name.
  Eg: stub path is /customer/:user_id/details -> and get.json file is

   "user_id": {{path.user_id}},
    .... 
    }```

- Other basic operations required : 
  - basic math operations
  - basic string operations
   

[esslushy]

May I try to add the basic operations. Seems like a good project.

[anroopak]

Yes please