Helium

This release of sgml-stream adds context lists to all methods.
This means you are now able to construct xhp elements in a pure context.

Breaking changes include:

• Context lists on methods may require adding context lists in subclasses.
• Subclasses that override init(): void with an impure method must fork their baseclass.
• Custom elements that directly extend RootElement must declare the context constant INITIALIZATION_CTX.
• FragElement->getFragChildren() are directly appended to the children of the embedding xhp class
  instead of being flattened first.