driver: support _time_format DSN query param

Building on the _pragma query param support added in 508747c9, support
a new _time_format query param.

If _time_format is set to "sqlite", written times use the time package
format "2006-01-02 15:04:05.999999999-07:00". This is the format
mattn/go-sqlite3 uses and is format 4 at
https://sqlite.org/lang_datefunc.html#time_values.

By default, times are formatted with time.Time.String. This maintains
compatibility with existing users of the driver.

Fixes #47

tcl_test.go

@@ -9,13 +9,14 @@ import (
 	"flag"
 	"fmt"
 	"io/ioutil"
-	"modernc.org/tcl"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
 	"testing"
+
+	"modernc.org/tcl"
 )
 
 var (
@@ -26,6 +27,10 @@ var (
 )
 
 func TestTclTest(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
+
 	blacklist := map[string]struct{}{}
 	switch runtime.GOARCH {
 	case "386", "arm":