suggestion: suggest `keyring` for storing google api

The discussion of how to acquire the google api key and use it are nice, thanks for the details.

Especially since you say

> Your API key is *private* ...

I wonder if it would be useful to suggest the use of the `keyring` package, where the user might do *once*:

```r
keyring::key_set("googleapi", "myusername")
# R prompts you for the apikey in a password-hidden-friendly popup
```

and then one could use

```r
register_google(key = keyring::key_get("googleapi", "myusername"))
```

either manually or within a script or `~/.Rprofile`. `keyring` works equally securely (IMO) on windows, linux, and macos ... the only place it doesn't work for me is when I `ssh` into a remote linux box, since there's no credentials app.

My use of `"googleapi"` is arbitrary, that's just a service name. One might argue for `("ggmap", "google_apikey")` or similar. The biggest impetus to standardize this would be if you intended to add `keyring`-awareness to your `register_google` function ... which might be justified.