NewHandler should return input if it is of type Handler

**Is your feature request related to a problem? Please describe.**
Just had an issue where we accidentally did something like this:
```go
h := lambda.NewHandler(...)
lambda.Start(h)
```
instead of this:
```go
h := lambda.NewHandler(...)
lambda.StartHandler(h)
```

This is caused by the fact that `Start` takes `interface{}`, so the compiler is unable to detect the error.

**Describe the solution you'd like**
`NewHandler` should check if the input implements `Handler` and if so return the input unchanged, possibly logging some kind of warning.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.