turn SQLite journaling off, for performance (gh-11)

Indexing the first 10000 articles from an nlwiki dump previously took:

real    1m17.655s
user    0m48.409s
sys     0m25.430s

Now:

real    0m46.042s
user    0m40.603s
sys     0m4.408s

semanticizest/parse_wikidump/createtables.sql

@@ -1,4 +1,5 @@
 pragma foreign_keys = on;
+pragma journal_mode = off;
 
 drop table if exists linkstats;
 drop table if exists ngrams;