config: Add 'ignore_config_files' parameter to ConfigFileFinder

The `--isolated` flag is passed into `MergedConfigParser.parse()` and
the module-level function `config.get_local_plugins()`.  Since both of
these places utilize the `ConfigFileFinder` object and isolation
pertains to how the `ConfigFileFinder` should behave with respect to
isolation, this incremental change more directly associates the
`ConfigFileFinder` and configuration file isolate.

src/flake8/main/application.py

@@ -335,7 +335,9 @@ class Application(object):
         prelim_opts, remaining_args = self.parse_preliminary_options(argv)
         flake8.configure_logging(prelim_opts.verbose, prelim_opts.output_file)
         config_finder = config.ConfigFileFinder(
-            self.program, prelim_opts.append_config
+            self.program,
+            prelim_opts.append_config,
+            ignore_config_files=prelim_opts.isolated,
         )
         self.find_plugins(
             config_finder, prelim_opts.config, prelim_opts.isolated

src/flake8/options/config.py

@@ -16,19 +16,26 @@ __all__ = ("ConfigFileFinder", "MergedConfigParser")
 class ConfigFileFinder(object):
     """Encapsulate the logic for finding and reading config files."""
 
-    def __init__(self, program_name, extra_config_files):
-        # type: (str, List[str]) -> None
+    def __init__(
+        self, program_name, extra_config_files, ignore_config_files=False
+    ):
+        # type: (str, List[str], bool) -> None
         """Initialize object to find config files.
 
         :param str program_name:
             Name of the current program (e.g., flake8).
         :param list extra_config_files:
             Extra configuration files specified by the user to read.
+        :param bool ignore_config_files:
+            Determine whether to ignore configuration files or not.
         """
         # The values of --append-config from the CLI
         extra_config_files = extra_config_files or []
         self.extra_config_files = utils.normalize_paths(extra_config_files)
 
+        # The value of --isolated from the CLI.
+        self.ignore_config_files = ignore_config_files
+
         # Platform specific settings
         self.is_windows = sys.platform == "win32"
         self.xdg_home = os.environ.get(