Added markdown and RST parsing to Article class. #6311

`Article` now has a `content_type` field for the same.
Part of #69.
I made some changes in tests with comments can you check that and add comments.

https://github.com/tarunbhardwaj/nereid-cms/blob/dc15f66f623d19eb5705038f1a05a7e0b50d6a6e/tests/test_cms.py#L287-L326