Support multiple instances of one provider type

[willemvd]

To be able to register the same provider twice we need to be able to override the provider name.
Don't want this to be a new func registered to the provider.Name but be inline with Client ( #123 )
Downside is that existing calls to provider.Name() will break (easy to replace with GetName() )

This will maybe also fix issue #126 ?

[willemvd]

No influence on #126

[markbates]

I'm not ok with a breaking API change. There needs to be a different way of doing this. Perhaps a setter? Or people could just wrap the provider in a type that reimplements the name function.

[willemvd]

Think you end up with something like this

		provider := github.New(clientID, clientSecret, "http://localhost:3000/auth/github/callback")
		provider.Name = func() string {
			return "github" // or any other name
		}

but when you have multiple types of providers this might get messy with lots of extra func definitions.
That is why I have suggested this approach, but keeping Name() with some extra SetName() might do the trick

[willemvd]

I will update the PR with that approach

[markbates]

So my problem now is none of the providers have a default name. We need to return a default name for each provider unless one is set. If this PR was merged it would break existing apps, unless I'm missing something.

[willemvd]

All the New methods set the default name in the Provider
example: https://github.com/willemvd/goth/blob/name-method/providers/amazon/amazon.go#L41

[willemvd]

Btw see a new conflict on intercom, will merge that

[markbates]

Ah! Awesome! I missed that. Sorry.

[willemvd]

NP 😄