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/fixtures/multi-plugin.js

@@ -0,0 +1,21 @@
+module.exports = {
+  verifyConditions(config, options, cb) {
+    cb();
+  },
+
+  getLastRelease(config, options, cb) {
+    cb();
+  },
+  analyzeCommits(config, options, cb) {
+    cb();
+  },
+  verifyRelease(config, options, cb) {
+    cb();
+  },
+  generateNotes(config, options, cb) {
+    cb();
+  },
+  publish(config, options, cb) {
+    cb();
+  },
+};

test/fixtures/plugin-error-b.js

@@ -1,3 +0,0 @@
-module.exports = function(config, options, cb) {
-  cb(new Error('b'));
-};

test/fixtures/plugin-noop.js

@@ -0,0 +1,3 @@
+module.exports = (config, options, cb) => {
+  cb(null);
+};

test/fixtures/plugin-result-a.js

@@ -1,3 +0,0 @@
-module.exports = function(config, options, cb) {
-  cb(null, 'a');
-};

test/fixtures/plugin-result-b.js

@@ -1,3 +0,0 @@
-module.exports = function(config, options, cb) {
-  cb(null, 'b');
-};