hermes.Theme has no support for exported fields

Given a Theme like this:

```go
type Theme struct {
Color string
Logo string
}

func (t *Theme) Name() string { ... }
func (t *Theme) HTMLTemplate() string { ... }
func (t *Theme) PlainTextTemplate() string { ... }
```

And calling hermes like this:

```go
h := hermes.Hermes{
Theme: new(Theme),
// ...
}
email := hermes.Email{
// ...
}
html, err = h.GenerateHTML(email)
if err != nil {
...
}
```

Will result int

```
panic: reflect: Field index out of range [recovered]
panic: reflect: Field index out of range
```

The reason for this is the exported fields, which are tried to be merged into `hermes.Default` theme which is of type `struct{}`. Therefore it's not possible to have exported fields on a struct implementing `hermes.Theme` interface.

Reproduce the panic with the exported-field Theme example and h.GenerateHTML(email), then inspect the theme-merging path involving hermes.Default. Done means a Theme with exported fields such as Color and Logo can be used without a reflection panic.

Written by the indexing model from the issue text.