Make `authenticate/2` more flexible

Currently, `Doorman.authenticate/2` is hardcoded to find the user by an email field. This is the only thing in the library that depends on it, and it would nice to have that be more flexible.

For example, you could search for username:

```
Doorman.authenticate(password, username: username)
```

Or admins:

```
Doorman.authenticate(password, admin: true, email: email)
```