Diag command

I wasted some time again helping someone figuring out that their oauth token wasn't valid anymore, and this isn't the only problem. Time and time again people ask for support and didn't do any due diligence. I'm not really blaming anyone though, most of the time they just don't know what to check for.

Anyway to help support efforts, and hopefully enable people to fix problems themselves more often, I thought scripting what is scriptable would be good. This runs for now like this:

```
$ composer diag
Checking platform settings: OK
Checking http connectivity: OK
Checking composer.json: OK
Checking github.com oauth access: OK
Checking composer version: OK
```

I'd welcome new ideas for automatic checks obviously, but that can be added later. Anyone think this is a bad idea?