Introduce the notion of context

[tidoust]

This update introduces the notion of a chunk context with wording and example taken from: https://github.com/w3c/cogai/blob/master/chunks-and-rules.md#statements-about-statements

It also describes the @context property and updates the "matching chunk" algorithm accordingly (algorithm was re-worded to better account for other corner cases. It still does not account for variables and for the ~ operator).

Linked to #25.

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[draggett]

You could note that chunks with @context won't match chunks without @context since the latter belong to the default context.

Rule sets and graph algorithms can be used to reason across contexts, including the possibility of context chains, for example, when reasoning about the reported beliefs of someone in a work of fiction.

[tidoust]

You could note that chunks with @context won't match chunks without @context since the latter belong to the default context.

Good point. I added a paragraph that clarifies how contexts affect chunk matching.