Enable using a custom text dataset in LSTM CLI

[c-w]

Currently the LSTM TensorflowJS example only supports a fixed set of preset text datasets. To enable users to easily experiment with their own datasets, this pull request enables passing arbitrary local text files to the training and generation scripts.

Additionally, I observed that the use of tf.loadModel in gen_node.js crashes since the method was renamed to tf.loadLayersModel in the tfjs 1.0 release. As such, this pull request also updates the model loading code to handle both the old and new method names.

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[pyu10055]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 2 of 1 approvals obtained (waiting on @caisq)