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_mskanji.csv

@@ -0,0 +1,4 @@
+<EFBFBD><EFBFBD><EFBFBD>O,<EFBFBD>N<EFBFBD><EFBFBD>,<EFBFBD>Z<EFBFBD><EFBFBD>
+<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>Y,30,<EFBFBD><EFBFBD><EFBFBD><EFBFBD>
+<EFBFBD>O<EFBFBD>؉p<EFBFBD>q,25,<EFBFBD><EFBFBD><EFBFBD><EFBFBD>
+<EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD>~,35,<EFBFBD><EFBFBD><EFBFBD>É<EFBFBD>