Add flair as an alternative

[bratao]

Changes proposed in this pull request:

I´m super sad that allennllp is veen deprecated. I was looking for alternatives to port my projects and I think that the best candidate is the Flair library. It have many of qualities of Allennlp such as the academic focus and the framework composability. I think it would be nice to add as an alternative.

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[dirkgr]

@bratao, if you want to step up as a maintainer of AllenNLP, we can do that. We can give you the necessary permissions now. It would give us enough time to do a soft transition until December.

[dirkgr]

Another comment: Did you consider https://github.com/allenai/tango as an upgrade? You would find the configuration language and the trainer quite familiar.

One way of looking at Tango is this: In AllenNLP, you always run three steps: Load data, train model, evaluate. In Tango, you can run those steps, but also other, custom steps, and you can run them in more complicated configurations than just in order.