master-master isn't possible with the in-build logical replication, but you can use BDR3 with pg10 to build a multi-master-cluster. BDR3 is a extension.
Multimaster replication solutions are business critical and so require rapid response and hot fixes for high availability and security issues. For BDR, this is only possible with close nurturing and ongoing support from the experts at 2ndQuadrant. 2ndQuadrant remains committed to active contribution of major new features and timely bug fixes to open source PostgreSQL. We will continue to contribute features from BDR to open source PostgreSQL at the pace the community allows.
Bucardo is an open-source project that has been around for 10 yrs+ providing multi-master support. http://bucardo.org