Add multitask bi-LSTM model

[astarostap]

Potential architecture is described in the "Model" section here: http://www.aclweb.org/anthology/E17-1005.

I think there's a PR for a normal bi-LSTM in the works by @nishithbsk , and when that's merged in we could potentially extend it to MTL?

[ajratner]

Yup once it's in, to start at least, it can just be fed in as the `input_layer`

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[astarostap]

Yup makes sense.

[bhancock8]

Snorkel MeTaL is moving into maintenance mode, as its features have been merged into Snorkel proper (https://github.com/snorkel-team/snorkel). If this feature request still applies, feel free to repost there!