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/pipeline.js

@@ -0,0 +1,19 @@
+const {identity} = require('lodash');
+const pReduce = require('p-reduce');
+
+module.exports = steps => async (input, getNextInput = identity) => {
+  const results = [];
+  await pReduce(
+    steps,
+    async (prevResult, nextStep) => {
+      // Call the next step with the input computed at the end of the previous iteration
+      const result = await nextStep(prevResult);
+      // Save intermediary result
+      results.push(result);
+      // Prepare input for next step, passing the result of the previous iteration and the current one
+      return getNextInput(prevResult, result);
+    },
+    input
+  );
+  return results;
+};