feat: Extract npm and github publish to plugins

- Add a new plugin type: `publish`
- Add support for multi-plugin. A plugin module can now return an object with a property for each plugin type
- Uses by default [npm](https://github.com/semantic-release/npm) and [github](https://github.com/semantic-release/github) in addition of Travis for the verify condition plugin
- Uses by default [npm](https://github.com/semantic-release/npm) and [github](https://github.com/semantic-release/github) for the publish plugin
- `gitTag` if one can be found is passed to `generateNotes` for both `lastRelease` and `nextRelease`
- `semantic-release` now verifies the plugin configuration (in the `release` property of `package.json`) and throws an error if it's invalid
- `semantic-release` now verifies each plugin output and will throw an error if a plugin returns an unexpected value.

BREAKING CHANGE: `githubToken`, `githubUrl` and `githubApiPathPrefix` have to be set at the [github](https://github.com/semantic-release/github) plugin level. They can be set via `GH_TOKEN`, `GH_URL` and `GH_PREFIX` environment variables.

BREAKING CHANGE: the `npm` parameter is not passed to any plugin anymore. Each plugin have to read `.npmrc` if they needs to (with https://github.com/kevva/npm-conf for example).

cli.js

@@ -10,22 +10,20 @@ module.exports = async () => {
     .name('semantic-release')
     .description('Run automated package publishing')
     .option('-b, --branch <branch>', 'Branch to release from')
-    .option('--github-token <token>', 'Token to authenticate with Github API')
-    .option('--github-url <url>', 'GitHub Enterprise endpoint')
-    .option('--github-api-path-prefix <prefix>', 'Prefix of the GitHub Enterprise endpoint')
     .option(
       '--verify-conditions <paths>',
-      'Comma separated list of paths or packages name for the verifyConditions plugin',
+      'Comma separated list of paths or packages name for the verifyConditions plugin(s)',
       list
     )
     .option('--get-last-release <path>', 'Path or package name for the getLastRelease plugin')
     .option('--analyze-commits <path>', 'Path or package name for the analyzeCommits plugin')
     .option(
       '--verify-release <paths>',
-      'Comma separated list of paths or packages name for the verifyRelease plugin',
+      'Comma separated list of paths or packages name for the verifyRelease plugin(s)',
       list
     )
     .option('--generate-notes <path>', 'Path or package name for the generateNotes plugin')
+    .option('--publish <paths>', 'Comma separated list of paths or packages name for the publish plugin(s)', list)
     .option('--debug', 'Output debugging information')
     .option(
       '-d, --dry-run',
@@ -43,7 +41,7 @@ module.exports = async () => {
       program.outputHelp();
       process.exitCode = 1;
     } else {
-      await require('./index')(program);
+      await require('./index')(program.opts());
     }
   } catch (err) {
     // If error is a SemanticReleaseError then it's an expected exception case (no release to be done, running on a PR etc..) and the cli will return with 0