v0.2.0

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.