Since the November Update, Microsoft has released a series of Insider Preview builds that so far didn't actually change a great deal that was visible. But the latest build, version 14291, changes all that, as it includes the first preview of the extension support in Edge. Microsoft has three extensions thus far: two it developed itself (one to perform automatic translation of foreign language pages, another to enable mouse gestures), and a ported version of the popular Reddit Enhancement Suite (RES).
Right now, installing extensions is rather clunky—when this is feature is released to end users, extensions will be included in the Windows Store and subject to some amount of vetting and oversight, but that's not plumbed in yet. Once installed, however, the extensions just seem to... work.