Add `endsWith` function

[danielfsousa]

I know we can use this function in ES6, but we already have a startsWithfunction, so why not add an endsWith too?

Here is the implementation: http://stackoverflow.com/questions/280634/endswith-in-javascript

By the way, thank you for taking your time to create this awesome course! 😄

[kentcdodds]

Sounds great! I'm looking forward to seeing your solution.