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/normalize.js

@@ -0,0 +1,34 @@
+const {promisify, inspect} = require('util');
+const {isString, isObject, isFunction, noop, cloneDeep} = require('lodash');
+const importFrom = require('import-from');
+
+module.exports = (pluginType, pluginConfig, logger, validator) => {
+  if (!pluginConfig) {
+    return noop;
+  }
+  const {path, ...config} = isString(pluginConfig) || isFunction(pluginConfig) ? {path: pluginConfig} : pluginConfig;
+  if (!isFunction(pluginConfig)) {
+    logger.log('Load plugin %s', path);
+  }
+  const plugin = isFunction(path) ? path : importFrom.silent(__dirname, path) || importFrom(process.cwd(), path);
+
+  let func;
+  if (isFunction(plugin)) {
+    func = promisify(plugin.bind(null, cloneDeep(config)));
+  } else if (isObject(plugin) && plugin[pluginType] && isFunction(plugin[pluginType])) {
+    func = promisify(plugin[pluginType].bind(null, cloneDeep(config)));
+  } else {
+    throw new Error(
+      `The ${pluginType} plugin must be a function, or an object with a function in the property ${pluginType}.`
+    );
+  }
+
+  return async input => {
+    const result = await func(cloneDeep(input));
+
+    if (validator && !validator.validator(result)) {
+      throw new Error(`${validator.message}. Received: ${inspect(result)}`);
+    }
+    return result;
+  };
+};