feat: support field tag prefix and custom field tag names

[donkeywon]

To avoid tag conflicts with other third-party libraries like env

1. support field tag prefix

   
   type Cfg struct {
   Path string `flag-short:"p" flag-long:"path" flag-description:"path to xxx"`
   }

cfg := &Cfg{} parser := flags.NewParser(cfg, flags.Default, flags.FlagTagPrefix("flag-")) parser.Parse()

2. support custom field tags name
```go
type Cfg struct {
    Path string `flag-custom-short:"p" long:"path" description:"path to xxx"`
}

flagTags := flags.NewFlagTags()
flagTags.Short = "flag-custom-short"
cfg := &Cfg{}
parser := flags.NewParser(cfg, flags.Default, flags.CustomFlagTags(flagTags))
parser.Parse()