autodoc: allow specifying values to global arguments

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.

doc/usage/extensions/autodoc.rst

@@ -353,8 +353,25 @@ There are also new config values that you can set:
 
    the directive will be interpreted as if only ``:members:`` was given.
 
+   You can also set `autodoc_default_flags` to a dictionary, mapping option
+   names to the values which can used in .rst files. For example::
+
+     autodoc_default_flags = {
+        'members': 'var1, var2',
+        'member-order': 'bysource',
+        'special-members': '__init__',
+        'undoc-members': None,
+    }
+
+    Setting ``None`` is equivalent to giving the option name in the list format
+    (i.e. it means "yes/true/on").
+
    .. versionadded:: 1.0
 
+   .. versionchanged:: 1.8
+
+      Specifying in dictionary format added.
+
 .. confval:: autodoc_docstring_signature
 
    Functions imported from C modules cannot be introspected, and therefore the