Fix issue #61: allow controlled vocabulary reference in linguistic types

[dan-ya]

This PR fixes issue #61.

What was wrong:

• There was no way to reference a controlled vocabulary (CONTROLLED_VOCABULARY_REF)
  when defining a LINGUISTIC_TYPE.

What this PR does:

• Adds an optional controlled_vocabulary argument to Elan.add_linguistic_type()
• When provided, the argument is written as CONTROLLED_VOCABULARY_REF in the ELAN file

API change:

• Elan.add_linguistic_type(..., controlled_vocabulary=None)

Closes #61

[dopefishh]

Thanks, I fixed the issues and merged your code #63