Cleanup and refactor, in preparation for plugin support. (#318)

* Work started moving converters to individual files.
* Significant cleanup and refactor.
* Moved everything to a packages subfolder.
* Added sample plugin.
* Added instructions to the README.md
* Bumped version, and added a note about compatibility.

packages/markitdown-sample-plugin/src/markitdown_sample_plugin/__init__.py

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: 2024-present Adam Fourney <adamfo@microsoft.com>
+#
+# SPDX-License-Identifier: MIT
+
+from ._plugin import __plugin_interface_version__, register_converters, RtfConverter
+from .__about__ import __version__
+
+__all__ = [
+    "__version__",
+    "__plugin_interface_version__",
+    "register_converters",
+    "RtfConverter",
+]