Add Random Alphanumeric String generation

[grantism]

Add the ability to generate a random String that contains alphanumeric characters. Users can also specify how many characters should be in the String.

[thellimist]

Could you modify the README.md?

Also I think it'd be nice if there were both String.random(5) and String.randomUnicode(10) which would include weird characters like ę, ç, æ, :1st_place_medal: (emojis) etc.