Turberfield-dialogue

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* Field lists at the document level are available via the `metadata` attribute of the model.
* Substitution references to Python values are properly resolved in the bodies of field lists.
* There is a new utility, `turberfield-dialogue` for producing a printable screenplay.
* The viewer module now registers all references with `turberfield.utils.assembly.Assembly`.
* The `Performer` class is now part of the public API.

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* Changelog fixes.

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* Substitution references are now permitted in the `resource` argument to
an FX directive.

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* `Turberfield.dialogue.performer` and matching tests implement the new Performer
class. This was first prototyped in the `bluemonday78` episode of Addison Arches.

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* `turberfield-rehearse` **--web** option works tolerably in Firefox.
* Added **strict** mode for casting a rehearsal.
* Interludes now see a sequence of folders they may **branch** to.
* State matching is hierarchical; '31' matches a criterion of '3'.

.. _turberfield-utils: https://github.com/tundish/turberfield-utils