feat: support multiple branches and distribution channels

- Allow to configure multiple branches to release from
- Allow to define a distribution channel associated with each branch
- Manage the availability on distribution channels based on git merges
- Support regular releases, maintenance releases and pre-releases
- Add the `addChannel` plugin step to make an existing release available on a different distribution channel

BREAKING CHANGE: the `branch` option has been removed in favor of `branches`

The new `branches` option expect either an Array or a single branch definition. To migrate your configuration:
- If you want to publish package from multiple branches, please the configuration documentation
- If you use the default configuration and want to publish only from `master`: nothing to change
- If you use the `branch` configuration and want to publish only from one branch: replace `branch` by `branches` (`"branch": "my-release-branch"` => `"branches": "my-release-branch"`)

.travis.yml

@@ -7,11 +7,14 @@ node_js:
   - 10
   - 8
 
-# Trigger a push build on master and greenkeeper branches + PRs build on every branches
+# Trigger a push build on release and greenkeeper branches + PRs build on every branches
 # Avoid double build on PRs (See https://github.com/travis-ci/travis-ci/issues/1147)
 branches:
   only:
     - master
+    - next
+    - beta
+    - /^\d+(.\d+)?.x$/
     - /^greenkeeper.*$/
 
 # Retry install on fail to avoid failing a build on network/disk/external errors