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).

test/helpers/git-utils.js

@@ -1,4 +1,3 @@
-import {mkdir} from 'fs-extra';
 import tempy from 'tempy';
 import execa from 'execa';
 import pMapSeries from 'p-map-series';
@@ -15,17 +14,13 @@ import pMapSeries from 'p-map-series';
 /**
  * Create a temporary git repository and change the current working directory to the repository root.
  *
- * @method gitCommits
- * @param {Array<Commit>} commits the created commits.
- *
  * @return {string} The path of the repository.
  */
 export async function gitRepo() {
   const dir = tempy.directory();
 
   process.chdir(dir);
-  await mkdir('git-templates');
-  await execa('git', ['init', '--template=./git-templates']);
+  await execa('git', ['init']);
   await gitCheckout('master');
   return dir;
 }