ignore colon with slash when split app_import_path

Flask currently supports importing app through a combination of module
path and app variable name, such as '/usr/app.py:my_app'. When the
module path contains a colon, it will conflict with this import way and
a `flask.cli.NoAppException` will be raised.

A file path on a Windows system may contain a colon followed by a slash.
So we solved this problem on Windows by ignoring the colon followed by a
slash when we split app_import_path.

Fix issue #2961.

flask/cli.py

@@ -369,7 +369,7 @@ class ScriptInfo(object):
             app = call_factory(self, self.create_app)
         else:
             if self.app_import_path:
-                path, name = (self.app_import_path.split(':', 1) + [None])[:2]
+                path, name = (re.split(r':(?![\\/])', self.app_import_path, 1) + [None])[:2]
                 import_name = prepare_import(path)
                 app = locate_app(self, import_name, name)
             else: