Reduce the number of imports needed for a basic program

[schmmd]

Here's some user feedback about the tutorial that applies to AllenNLP in general:

• the huge imports section where many components are important from many different packages can be a big "turn off" for adoption, imo, as it requires remembering many different things (which modules to import and where to import them from) just to get a very basic program running. I think adding a meta-package (i.e. allennlp.all) which you can import everything from will make things much more user friendly also for quick-and-dirty stuff.

[DeNeutoy]

We can simplify this considerably by doing from allennlp import data and then using data.Field in the tutorial. This should reduce the initial cognitive load.

[schmmd]

We're going to wait and see if we get additional similar feedback before we take action here