Added characters class method to Faker::Lorem

So I found myself want to speed up some tests I'm running by switching from __faker__ to __ffaker__. However there are many, many tests which leverage __faker__'s Faker::Lorem.characters method. Rather than throw in the towel I figured it would be simpler to add the characters method to __ffaker__ hence the pull request.

I added a plaintext _characters_ file to the data folder to stay idiomatic with the project and added the characters method along with the accompanying tests. Look's like it all checks out. I benchmarked it  and the new method runs about twice as fast as the __faker__ version.

Cheers