Add Document Builder type

[aThorp96]

Hello! I implemented these features when using your library but I think they might serve better purpose as part of the library.

The two features are

1. A DocumentBuilder which allows you to pragmatically construct gemini files line-by line with additional support for headers and footers ~2. A RequireInputHandler which allows the user to provide a custom prompt (META) on the response.~ Moving to a separate PR per request