Previously, users could specify a *list* of flags in their config files.
The flags were directive names that would otherwise be present in the
.rst files. However, as a list, it was not possible to specify values
to those flags, which *is* possible in .rst files.
For example, in .rst you could say
:special-members: __init__, __iter__
And this would cause autodoc to generate documents for these methods that
it would otherwise ignore.
This commit changes the config option to instead accept a dictionary.
This is a dictionary whose keys can contain the same flag-names as before,
but whose values can contain the arguments as seen in .rst files.
The old list is still supported, for backwards compatibility, but the data
is transformed into a dictionary when the user's config is loaded.
