Add parsing for special Semantic Pointers.

[jgosmann]

Motivation and context:
Addresses #176.

Allows spa.sym.Zero, vocab['Identity'], and vocab.parse('AbsorbingElement') to access special Semantic Pointers. These names won't be listed in vocab.keys() or vocab.vectors.

Interactions with other PRs:
none

How has this been tested?
added a test

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

[Seanny123]

Still lookin' good.