New flag interface support (go1.14)

```
# github.com/ejunjsh/dl
../go/src/github.com/ejunjsh/dl/main.go:25:22: cannot use cli.StringSliceFlag literal (type cli.StringSliceFlag) as type cli.Flag in slice literal:
	cli.StringSliceFlag does not implement cli.Flag (Apply method has pointer receiver)
../go/src/github.com/ejunjsh/dl/main.go:41:17: cannot range over c.Args() (type cli.Args)
```