Support varargs

Example:

``` js
/** @param {...ParamType} var_args */
var fnVarArgs = function(var_args) {
  arguments; //: [ParamType]
  var_args; //: ...ParamType
};
fnVarArgs; //: fn(var_args: ...ParamType)
```

This will always be a bit of a hack because we'd have to apply the varargs type to the entire `arguments` variable, but it would be serviceable.
