Auto-detect org in `config validate`

[glenjamin]

What problem does this feature solve?:

> circleci config validate
Error: config compilation contains errors: config compilation contains errors:
	- Cannot find geckoboard/deployment-tooling@1 in the orb registry. Check that the namespace, orb name and version are correct.

> circleci config validate -o gh/geckoboard
Config file at .circleci/config.yml is valid.

Validation requires the caller to specify the org that the config will be run under.

This information could be inferred in the majority of cases by looking at the origin remote in .git/config - which would be a nice quality-of-life improvement