Database storage and runtime caching

Source files are loaded to the Filter before use. When loaded, the jurisdiction, category key, text key and value of each item is recorded in a set of (normalized) SQLite database tables, and associated with a particular citation style.

The Filter provides a primitive method getAbbreviation() that returns matching values from the database. In citeproc-js, this is wrapped in a loadAbbreviation() method that first hits a cache object at citeproc.transform.abbrevs before resorting to a database call.