feat: Make semantic-release language agnostic

- Do not rely on `package.json` anymore
- Use `cosmiconfig` to load the configation. `semantic-release` can be configured:
  - via CLI options (including plugin names but not plugin options)
  - in the `release` property of `package.json` (as before)
  - in a `.releaserc.yml` or `.releaserc.js` or `.releaserc.js` or `release.config.js` file
  - in a `.releaserc` file containing `json`, `yaml` or `javascript` module
- Add the `repositoryUrl` options (used across `semantic-release` and plugins). The value is determined from CLi option, or option configuration, or package.json or the git remote url
- Verifies that `semantic-release` runs from a git repository
- `pkg` and `env` are not passed to plugin anymore
- `semantic-release` can be run both locally and globally. If ran globally with non default plugins, the plugins can be installed both globally or locally.

BREAKING CHANGE: `pkg` and `env` are not passed to plugin anymore.
Plugins relying on a `package.json` must verify the presence of a valid `package.json` and load it.
Plugins can use `process.env` instead of `env`.

lib/git.js

@@ -72,4 +72,20 @@ async function gitHead() {
   }
 }
 
-module.exports = {gitTagHead, gitCommitTag, isCommitInHistory, unshallow, gitHead};
+/**
+ * @return {string|null} The value of the remote git URL.
+ */
+async function repoUrl() {
+  return (await execa.stdout('git', ['remote', 'get-url', 'origin'], {reject: false})) || null;
+}
+
+/**
+ * @return {Boolean} `true` if the current working directory is in a git repository, `false` otherwise.
+ */
+async function isGitRepo() {
+  const shell = await execa('git', ['rev-parse', '--git-dir'], {reject: false});
+  debugShell('Check if the current working directory is a git repository', shell, debug);
+  return shell.code === 0;
+}
+
+module.exports = {gitTagHead, gitCommitTag, isCommitInHistory, unshallow, gitHead, repoUrl, isGitRepo};