Provide support for key prefixing when navigating into structures

Let's say I have a database struct, maybe provided by my database internal package

```go
type DatabaseConfig struct {
  Host  string  `key:"HOST"`,
  Port  int       `key:"PORT"`,
  //...
}
```

I may want to introduce two of them

```go
type MyProgramConfig struct {
   UserDatabase    DatabaseConfig  `keyPrefix:"USER_"`
   ProductDatabase  DatabaseConfig  `keyPrefix:"PRODUCT_"`
}
```