Option to completely ignore voids

[pscheidler]

Voids are currently kept as small, unused objects. They could be removed

[StokesMIDE]

Clarification:

• "Ignore Voids" would be an argument for Schema.load() and affect the resulting Document.
• Void elements would not show up when either the Document or Master elements are iterated. They would not get cached, nor show up in the total returned by len().