Add prebuilt index for ACL Anthology

[lilyjge]

Lucene inverted BM25 index for ACL Anthology. To make sure all fields are indexed without getting flagged as 'empty', empty abstracts and authors have 'n/a' as placeholders. Fields are title, contents (abstract), authors (name, variations separated by commas; institution after name after semicolon. authors separated by periods), venue (name of conference/journal), and year. IDs are the 'bibkey', guaranteed to be unique across the Anthology.