Improvements to `Content` class functionality

[robmoffat]

Feature Request

Description of Problem:

Manipulating content is a bit of a pain. If you want to remove the first word from a paragraph, you're out of luck.

Potential Solutions:

Content.removeNth() : returns a new Content.
Content.remove(object) : returns a new Content

List extends Content {}   # Handle lists in chats

OrderedList extends List {}
UnorderedList extends List {}
ListItem extends Pararaph {}