Create standalone functions for API methods

[atc0005]

As suggested by @nmaupu on GH-93:

I see a possible improvement though: it would be a great idea to have a singleton that one can use directly (other libraries do that as well, see viper for example: https://github.com/spf13/viper/blob/master/viper.go#L63)
So no need to even create a NewClient() if your usage is the default one 👍 Calls could look like the following:

import teams github.com/atc0005/go-teams-notify
[...]
func main() {
  _ := teams.SendWithContext(context.TODO(), webhook, msg)
}

I agree with the reasoning. Having this available would make getting started with the library even easier.