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).

lib/plugins/index.js

@@ -0,0 +1,24 @@
+const {isArray} = require('lodash');
+const DEFINITIONS = require('./definitions');
+const pipeline = require('./pipeline');
+const normalize = require('./normalize');
+
+module.exports = (options, logger) =>
+  Object.keys(DEFINITIONS).reduce((plugins, pluginType) => {
+    const {config, output, default: def} = DEFINITIONS[pluginType];
+    let pluginConfs;
+    if (options[pluginType]) {
+      if (config && !config.validator(options[pluginType])) {
+        throw new Error(config.message);
+      }
+      pluginConfs = options[pluginType];
+    } else {
+      pluginConfs = def;
+    }
+
+    plugins[pluginType] = isArray(pluginConfs)
+      ? pipeline(pluginConfs.map(conf => normalize(pluginType, conf, logger, output)))
+      : normalize(pluginType, pluginConfs, logger, output);
+
+    return plugins;
+  }, {});