Tokenize arrays of strings

[ghost]

Is your feature request related to a problem? Please describe.

The BaseSampler will handle arrays of tensors elegantly. However, these arrays only come to be if you manually construct them yourself.

Describe the solution you'd like

To be able to have an array-as-one-observation, and for the TextField to handle that gracefully (then pass it along to the BaseSampler).

Additional context

Models that embed a large sequence of context utterances, where simply joining those utterances into one string makes for one really long string.

[jeremyasapp]

Done in #255