Add Go API for virtual tables

See https://www.sqlite.org/vtab.html for more details.

This work was started from
https://github.com/gwenn/gosqlite/blob/master/vtab.{c,go} and adds:

- Porting the API to go-sqlite3 APIs.
- Support for >= Go 1.6 without requiring the `cgocheck` flag to be changed.
- Filling out the unfinished callback functions for the `Vtable` struct.
- A simple `Context` API layer for ease of use when adding modules.

Tests are included.

sqlite3.go

@@ -627,11 +627,11 @@ func (d *SQLiteDriver) Open(dsn string) (driver.Conn, error) {
 
 // Close the connection.
 func (c *SQLiteConn) Close() error {
-	deleteHandles(c)
 	rv := C.sqlite3_close_v2(c.db)
 	if rv != C.SQLITE_OK {
 		return c.lastError()
 	}
+	deleteHandles(c)
 	c.db = nil
 	runtime.SetFinalizer(c, nil)
 	return nil