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).

package.json

@@ -18,7 +18,8 @@
     "@semantic-release/commit-analyzer": "^4.0.0",
     "@semantic-release/condition-travis": "^6.0.0",
     "@semantic-release/error": "^2.1.0",
-    "@semantic-release/last-release-npm": "^2.0.0",
+    "@semantic-release/github": "^1.0.0",
+    "@semantic-release/npm": "^1.0.0",
     "@semantic-release/release-notes-generator": "^5.0.0",
     "chalk": "^2.3.0",
     "commander": "^2.11.0",
@@ -26,18 +27,13 @@
     "execa": "^0.8.0",
     "fs-extra": "^4.0.2",
     "get-stream": "^3.0.0",
-    "git-head": "^1.2.1",
     "git-log-parser": "^1.2.0",
-    "github": "^12.0.0",
+    "import-from": "^2.1.0",
     "lodash": "^4.0.0",
     "marked": "^0.3.6",
     "marked-terminal": "^2.0.0",
-    "nerf-dart": "^1.0.0",
     "normalize-package-data": "^2.3.4",
-    "npm-conf": "^1.1.2",
-    "p-series": "^1.0.0",
-    "parse-github-repo-url": "^1.3.0",
-    "require-relative": "^0.8.7",
+    "p-reduce": "^1.0.0",
     "semver": "^5.4.1"
   },
   "devDependencies": {