Support adding extensions locally for dev

Created by: sqs

From @mhewett (https://github.com/sourcegraph/sourcegraph-extension-api/issues/92):

It would be nice to be able to test an extension on a local Sourcegraph installation before publishing it to the world. Can we have a way to install an extension in a local installation, including Docker-based installations?

(Moved here because it would be implemented here.)

Related: https://github.com/sourcegraph/sourcegraph/issues/480 (we will do that before this)