feat: support sharable configuration

Adds the options `extends`, which can be defined via configuration file or CLI arguments to a single path or an array of paths of shareable configuration.
A shareable configuration is a file or a module that can be loaded with `require`.
Options is defined by merging in the following order of priority:
- CLI/API
- Configuration file
- Shareable configuration (from right to left)

Options set in a shareable configuration can be unset by setting it to `null` or `undefined` in the main configuration file. If a default value applies to this property it will be used.

lib/git.js

@@ -73,10 +73,10 @@ async function gitHead() {
 }
 
 /**
- * @return {string|null} The value of the remote git URL.
+ * @return {string|undefined} The value of the remote git URL.
  */
 async function repoUrl() {
-  return (await execa.stdout('git', ['remote', 'get-url', 'origin'], {reject: false})) || null;
+  return (await execa.stdout('git', ['remote', 'get-url', 'origin'], {reject: false})) || undefined;
 }
 
 /**