Add configured?/1

[mnishiguchi]

This is an enhancement adding one convenience function that checks if a WLAN interface is already configured.

This check is used in at least a few Nerves-related projects, such as Circuits Quickstart and Nerves Livebook. Since the check is related to WLAN, it would be nice to place the decision-making logic for the check here in VintageNetWiFi.

What do you all think? 🤔

Here are some related commits:

• https://github.com/elixir-circuits/circuits_quickstart/commit/c2608c8be77a917913584f11272179f6f5dfee9b
• https://github.com/nerves-livebook/nerves_livebook/commit/713bff6bcfac333c752bd9ec4718cf5422854926

There can be a better name for the function. configured? is kind of ambiguous...

[fhunleth]

I think this is a good idea. Let me think about the name a little bit later on today.