Metadata: Reading System Requirements

This is a possible requirement for formalizing different aspects of the document's content requirements of a reading system's features. These features may include:
- has graphics
- has graphics which occur inline with text
- has (floating) page numbers
- has hyperlinks
- has hyperlinks to external resources (e.g. another book, website etc)

Benefits:
- Libraries or marketplaces could allow filtering based on system capabilities
- Users would have prior knowledge of what experience to expect with their reading system for the document
- Reading systems could advise the user about unavailable features
- Reading systems could establish fallback handling

Drawbacks:
- Requires authors or authoring tools to preprocess the content
- Prior systems (e.g. ePub) have not succeeded in getting traction on such system requirements

There are two possible paths forward to technically implementing this:
1) **Explicit**: Formalize a `requirements` entry, with a list of required features;
2) **Implicit**: Advertise the document features in other metadata entries, so the reading system can infer the match with requirements.

As an example of the implicit case, the graphics-present entry (#118), instead of being a boolean, could describe the nature of graphics present, such as whether they are tactile, whether the graphics are inline with text or on a separate page, etc.

This issue requires further discussion on which requirements are critical for the MVP.

## References:
- See #48 for detailed discussion
- #118