Would like for SimpleEmailMessage to include Tags

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I find a struct that solves 99% of my problems, but then falls just short.

**Describe the solution you'd like**
When processing SES events in golang, I'd like for the [SimpleEmailMessage](https://github.com/aws/aws-lambda-go/blob/3afc312542924bc260516dd416c66d43f990390c/events/ses.go#L21) struct to include `Tags`.

**Describe alternatives you've considered**
I will need to print out an example event and take it to a golang JSON parser to build a custom struct.

**Additional context**
Just update to this? I can handle the type conversion/parsing from there.
```
type SimpleEmailMessage struct {
CommonHeaders SimpleEmailCommonHeaders `json:"commonHeaders"`
Source string `json:"source"`
Timestamp time.Time `json:"timestamp"`
Destination []string `json:"destination"`
Headers []SimpleEmailHeader `json:"headers"`
HeadersTruncated bool `json:"headersTruncated"`
MessageID string `json:"messageId"`
Tags map[string][]string `json:"tags"`
}
```