Updated the Sightly specification to include JSON and XML contexts, and allow passthrough of strings when using array join

[vladbailescu]

This PR:

• adds JSON context (closes #5)
• adds XML context (closes #6)
• allows string passthrough when using array join (#7 also requires a TCK update)

[raducotescu]

Defined the behaviour of the join option for strings in f2afcf2. Version 1.1 will not provide support for JSON / XML XSS protection contexts.