config: add new 'docstring-code-format' and 'docstring-code-line-length' knobs
This commit does the plumbing to make a new formatting option,
'docstring-code-format', available in the configuration for end users.
It is disabled by default (opt-in). It is opt-in at least initially to
reflect a conservative posture. The intent is to make it opt-out at
some point in the future.
This also adds a compansion option, 'docstring-code-line-length', that
permits setting a line length for reformatted code examples that is
distinct from the global setting. Its default value is 'dynamic', which
means reformatted code will respect the global line width, regardless of
the indent level of the enclosing docstring.