refactor: Use ES6, Test with AVA

- Use async/await instead of callbacks
- Use execa to run command line
- Use AVA for tests
- Add several assertions in the unit tests
- Add documentation (comments) in the tests
- Run tests with a real git repo instead of mocking child_process and add test helpers to create repos, commits and checkout
- Simplify test directory structure
- Simplify code readability (mostly with async/await)
- Use eslint for for linting, prettier for formatting

test/fixtures/plugin-error-a.js

@@ -0,0 +1,3 @@
+module.exports = function(config, options, cb) {
+  cb(new Error('a'));
+};

test/fixtures/plugin-error-b.js

@@ -0,0 +1,3 @@
+module.exports = function(config, options, cb) {
+  cb(new Error('b'));
+};

test/fixtures/plugin-result-a.js

@@ -0,0 +1,3 @@
+module.exports = function(config, options, cb) {
+  cb(null, 'a');
+};

test/fixtures/plugin-result-b.js

@@ -0,0 +1,3 @@
+module.exports = function(config, options, cb) {
+  cb(null, 'b');
+};

test/fixtures/plugin-result-config.js

@@ -0,0 +1,3 @@
+module.exports = function(pluginConfig, options, cb) {
+  cb(null, {pluginConfig, options});
+};