Support black's --experimental-string-processing flag

[feluelle]

It would be great if blacken-docs would also allow using the --experimental-string-processing to split lines in the docstring if they reach the line length limit.

[adamchainz]

Black 22.1.0 deprecated --experimental-string-processing and rolled it into --preview, so I've added support for passing through --preview in #273.

Note though that it's probably better to put the option into your configuration, like pyproject.toml:

[tool.black]
preview = true

This way the option will apply to both black and blacken-docs with no further options.