Refactor: Add Config.read() as a constructor

To simplify Config.__init__() method, this separates conf.py parsing
feature to Config.read() method.
This allows to instantiate config object simply.

tests/test_directive_code.py

@@ -17,7 +17,7 @@ from sphinx.config import Config
 from sphinx.directives.code import LiteralIncludeReader
 from sphinx.testing.util import etree_parse
 
-DUMMY_CONFIG = Config(None, None, {}, '')
+DUMMY_CONFIG = Config({}, {})
 
 
 @pytest.fixture(scope='module')