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/tests/test_files/test.json

@@ -0,0 +1,10 @@
+{
+    "key1": "string_value",
+    "key2": 1234,
+    "key3": [
+        "list_value1",
+        "list_value2"
+    ],
+    "5b64c88c-b3c3-4510-bcb8-da0b200602d8": "uuid_key",
+    "uuid_value": "9700dc99-6685-40b4-9a3a-5e406dcb37f3"
+}