Add support for "string" type tensors

[bileschi]

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TensorFlow.js version

0.11.6

Browser version

irrelevant

Describe the problem or feature request

Support Tensors of dType 'string' and ops associated with that dtype, such as those here:
https://www.tensorflow.org/api_guides/python/string_ops
e.g.,
tf.string_to_hash_bucket
tf.string_split
tf.string_to_number

Code to reproduce the bug / link to feature request

t = tf.tensor1d(['hi'])

[dsmilkov]

String tensor and basic functionality added in tensorflow/tfjs-core#1408, thus closing this issue. We can open separate issues per op.