apidoc: read options from SPHINX_APIDOC_OPTIONS envvar if set

If the SPHINX_APIDOC_OPTIONS environment variable exists, read
OPTIONS from there, otherwise use the existing default options.
This allows some configurability of sphinx-apidoc.

sphinx/apidoc.py

@@ -20,6 +20,9 @@ import optparse
 from os import path
 
 # automodule options
+if 'SPHINX_APIDOC_OPTIONS' in os.environ:
+    OPTIONS = os.environ['SPHINX_APIDOC_OPTIONS'].split(',')
+else:
     OPTIONS = [
         'members',
         'undoc-members',