add support for defining an "eponymous only" virtual table (#885)

* add support for defining an "eponymous only" virtual table

As suggested here: https://github.com/mattn/go-sqlite3/issues/846#issuecomment-736206222

* add an example of an eponymous only vtab module

* add a test case for an eponymous only vtab module

_example/vtable_eponymous_only/main.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"database/sql"
+	"fmt"
+	"log"
+
+	"github.com/mattn/go-sqlite3"
+)
+
+func main() {
+	sql.Register("sqlite3_with_extensions", &sqlite3.SQLiteDriver{
+		ConnectHook: func(conn *sqlite3.SQLiteConn) error {
+			return conn.CreateModule("series", &seriesModule{})
+		},
+	})
+	db, err := sql.Open("sqlite3_with_extensions", ":memory:")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer db.Close()
+
+	rows, err := db.Query("select * from series")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var value int
+		rows.Scan(&value)
+		fmt.Printf("value: %d\n", value)
+	}
+}