Version checker

Nice to have feature:

Display current and latest release versions. Ideally only check on certain commands.

`slcli --version`
`slcli -v`
or any command that throws an uncaught exception.

```
slcli --version
Current: slcli (SoftLayer Command-line), version 5.8.0
Latest:   slcli (SoftLayer Command-line), version 5.8.5
```

Something like that.