New syntax for constraints

[lucapette]

Based on a conversation we had on slack. @jorinvo proposed we change the way constrains/range (we should probably just call these "options" so that it's generic enough to work for different generators) this way:

$ fakedata user=enum,John..Joe..James age=int,1..100 # current syntax
$
$ 
$ fakedata user=enum:John,Joe,James age=int:1,100 # new syntax

I think it looks great and I'd proceed right away. The feature requires us to:

• Change to the way we parse the data
• Rename constraints to something better (Options maybe?)
• Update all the integration/unit tests (as we change behavior, we break all the tests :))
• Update all the examples in the README (which I'd like to test via integration tests at some point but that's a different conversation

It's a very exciting change as it leads to a much simpler (therefore better UI). I can take care of it unless any of you would like to give it a try.

[lucapette]

Just started working on it! All the tests are failing 🎉

[lucapette]

Done in #48