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"`)

test/plugins/normalize.test.js

@@ -152,6 +152,24 @@ test('Wrap "publish" plugin in a function that validate the output of the plugin
   t.regex(error.details, /2/);
 });
 
+test('Wrap "addChannel" plugin in a function that validate the output of the plugin', async t => {
+  const addChannel = stub().resolves(2);
+  const plugin = normalize(
+    {cwd, options: {}, stderr: t.context.stderr, logger: t.context.logger},
+    'addChannel',
+    addChannel,
+    {}
+  );
+
+  const error = await t.throws(plugin({options: {}}));
+
+  t.is(error.code, 'EADDCHANNELOUTPUT');
+  t.is(error.name, 'SemanticReleaseError');
+  t.truthy(error.message);
+  t.truthy(error.details);
+  t.regex(error.details, /2/);
+});
+
 test('Plugin is called with "pluginConfig" (with object definition) and input', async t => {
   const pluginFunction = stub().resolves();
   const pluginConf = {path: pluginFunction, conf: 'confValue'};