Add string interpolation

Unlike [#3](https://github.com/sindresorhus/multiline/pull/3) this requires the user to send in an object with the variables, and doesn't modify the behavior of the original function if no extra argument is passed. I guess it still might be a bit outside the project's scope, but if it's something that seems suitable I can add a few tests for it.

Example:
````javascript
var testVar = 'Hi!';
var testString = multiline(function() {/*
Variable testVar is ${testVar}.
Another variable is ${other}.
*/}, {
  testVar: testVar,
  other: 'Something else.'
});
````