Add support to also track availability

[m-rtijn]

As an optional feature, a crawler may add availability information to a newly crawled price.

Availability information will be an extra property of the Price data model class. Possible values for the availability property are:

• available directly from own stock ("uit voorraad leverbaar")
• available from suppliers
• not available, new stock expected
• not available (no information about future restocking)
• will never be available
• no availability information (null)