_search= queries now correctly escaped, fixes #651

Queries with reserved words or characters according to the SQLite
FTS5 query language could cause errors.

Queries are now escaped like so:

    dog cat => "dog" "cat"

datasette/sql_functions.py

@@ -0,0 +1,7 @@
+from datasette import hookimpl
+from datasette.utils import escape_fts
+
+
+@hookimpl
+def prepare_connection(conn):
+    conn.create_function("escape_fts", 1, escape_fts)