Added String Interpolation

You can interpolate tags within a string!

    chance.interpolate("Dear `prefix` `last`, you rolled a `rpg(\"2d5\",{\"sum\":true})`. You win `dollar()`!");
    => Dear Mrs. Chavez, you rolled a 9. You win $4639.83!

To avoid using `eval(tag)`, a utility function called  `_parseFunction(String)` parses the tag.
Unfortunately, the arguments of the function must be JSON compliant (e.g. `rpg('2d5',{sum:true})` won't work.)

__Supported tags__

    Function name: `bool`
    Function with empty args:  `bool()`
    Function with args  `bool({"likelihood":30})` // must be JSON compliant
    Function with properties  `socialuser.network`