Support Embedded Structs

If you have a parent struct that has an embedded child struct, the fields of the child struct will not get their SOQL fields marshaled.

```
type Parent struct {
Child
}
type Child struct {
Field string `soql:"selectColumn,fieldName=My_Field__c"`
}
```

Marshalling a query including the parent will omit `Field` despite it being a directly accessible member of `Parent`. I believe this is incongruous with how JSON serialization works in Golang.

I'd be happy to pick this up as an external contribution with a few pointers!