Mobiledoc

Removed the code that checks if the current atom is already included.

Added a new method, `add_markdown_from_html` that uses the markdownify to convert HTML strings into Markdown.

Fixed the issue that `__init__.py` file on the `mobiledoc` pointing at the variable, `mobiledoc` instead of `MobileDoc` class.
Added tests

Added more methods as follows:
- **`add_button(text: str, url: str, alignment:str = "center")`**: Adds a button to mobiledoc.
- **`add_HTML(self, html:str):`**: Adds raw HTML card to mobiledoc.
- **`add_markdown(self, markdown:str):`**: Adds raw markdown card to mobiledoc.
- **`add_file(self, url: str, filename: str, filetitle: str, filesize: int, filecaption: str = ""):`**: Adds a file to mobiledoc.
- **`def add_callout(self, text: str, emoji: str = "", color: str = "accent"):`**: Adds a callout to mobiledoc.

This is the starting point of this package.

This supports some of the basic formatting options in mobiledoc format such as:
- Text formatting (bold, code, italic, underline, subscript, superscript, strikethrough, and hyperlink.)
- Adding images
- Adding dividers
- Adding custom data
And other APIs that allows to fetch the current status of the mobiledoc.