'whoami' to get current username

I want a feature to grab the current GitHub username from shell so that it can easily be piped into other processes and tools (e.g. filling a `package.json` with the correct github repos). 

Usage:

```gh user --whoami```

The name was chosen for parity with bash `whoami`. 

It would also be great if there was a way to suppress logging so that this command doesn't emit unnecessary logs.