New Check: presence of @impl if there is a @behaviour

[marcandre]

What do you want Credo to do?

While Elixir does not require @impl statements, I would like that the presence of @behaviour Anything implies the presence, somewhere in the code, of a @impl statement. Note that if there is at least one @impl statement, then Elixir enforces that they are all decorated.

In this check or separate check, I'd like that @impl true be forbidden

Which existing behaviour would change?

Note: this is similar to #13, but more basic and can be implemented statically.

[marcandre]

I can provide a PR for this if this is accepted.

[rrrene]

After our conversation yesterday, I would absolutely love to see what you come up with 👍