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"`)

lib/definitions/plugins.js

@@ -71,8 +71,22 @@ module.exports = {
     pipelineConfig: () => ({
       // Add `nextRelease` and plugin properties to published release
       transform: (release, step, {nextRelease}) => ({
-        ...(isPlainObject(release) ? release : {}),
         ...nextRelease,
+        ...(release || {}),
+        ...step,
+      }),
+    }),
+  },
+  addChannel: {
+    default: ['@semantic-release/npm', '@semantic-release/github'],
+    required: false,
+    dryRun: false,
+    outputValidator: output => !output || isPlainObject(output),
+    pipelineConfig: () => ({
+      // Add `nextRelease` and plugin properties to published release
+      transform: (release, step, {nextRelease}) => ({
+        ...nextRelease,
+        ...(release || {}),
         ...step,
       }),
     }),