Different JSON structures from providers

[tohagan]

Each of the providers produces slightly different JSON structures. So what I think this lib needs is an optional transform function per provider to convert the JSON into a set of common fields.
Some of the providers DO produce similar results so they could become the defacto standard.

[roryrjb]

@tohagan yes very good point. I'm happy to accept a pull request for this, otherwise I can probably work on it this weekend.

[tohagan]

Some also now require an access key that you could include as an input. The result they return can depend on whether you present a key. Of course each provider wants their own key so again you need to provide a customisation (key + transform fn) per provider.

[roryrjb]

The different structure issue is closed in #14, with an initial implementation. Regarding the need for API keys, I've trimmed the default provider list so none require API keys, but the actual implementation needs to be addressed, I've opened #15 for this. As always PRs are more than welcome! 👍