Support es modules (#1583)

Ref https://github.com/svg/svgo/issues/1579

In config of course. Projects with type:module can now use modules to
export config

```js
export default {
  plugins: []
}
```

Also added support for resolving svgo.config.mjs and svgo.config.cjs.

Moved loadConfig tests to svgo-node tests.

mjs test is skipped for now in node 10, just don't use modules there

lib/svgo-node.test.js

@@ -5,7 +5,8 @@
  */
 
 const os = require('os');
-const { optimize } = require('./svgo-node.js');
+const path = require('path');
+const { optimize, loadConfig } = require('./svgo-node.js');
 
 const describeLF = os.EOL === '\r\n' ? describe.skip : describe;
 const describeCRLF = os.EOL === '\r\n' ? describe : describe.skip;
@@ -123,3 +124,94 @@ describeCRLF('with CRLF line-endings', () => {
     );
   });
 });
+
+describe('loadConfig', () => {
+  const cwd = process.cwd();
+  const fixtures = path.join(cwd, './test/fixtures/config-loader');
+
+  test('loads by absolute path', async () => {
+    expect(await loadConfig(path.join(fixtures, 'one/two/config.js'))).toEqual({
+      plugins: [],
+    });
+  });
+
+  test('loads by relative path to cwd', async () => {
+    const config = await loadConfig('one/two/config.js', fixtures);
+    expect(config).toEqual({ plugins: [] });
+  });
+
+  test('searches in cwd and up', async () => {
+    expect(await loadConfig(null, path.join(fixtures, 'one/two'))).toEqual({
+      plugins: [],
+    });
+    expect(
+      await loadConfig(null, path.join(cwd, './test/fixtures/missing'))
+    ).toEqual(null);
+    // TODO remove check in v3
+    if (process.version.startsWith('v10.') === false) {
+      expect(await loadConfig(null, path.join(fixtures, 'mjs'))).toEqual({
+        plugins: ['mjs'],
+      });
+    }
+    expect(await loadConfig(null, path.join(fixtures, 'cjs'))).toEqual({
+      plugins: ['cjs'],
+    });
+  });
+
+  test('fails when specified config does not exist', async () => {
+    try {
+      await loadConfig('{}');
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/Cannot find module/);
+    }
+  });
+
+  test('fails when exported config not an object', async () => {
+    try {
+      await loadConfig(path.join(fixtures, 'invalid-null.js'));
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/Invalid config file/);
+    }
+    try {
+      await loadConfig(path.join(fixtures, 'invalid-array.js'));
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/Invalid config file/);
+    }
+    try {
+      await loadConfig(path.join(fixtures, 'invalid-string.js'));
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/Invalid config file/);
+    }
+  });
+
+  test('handles runtime errors properly', async () => {
+    try {
+      await loadConfig(path.join(fixtures, 'invalid-runtime.js'));
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/plugins is not defined/);
+    }
+    // TODO remove check in v3
+    if (process.version.startsWith('v10.') === false) {
+      try {
+        await loadConfig(path.join(fixtures, 'invalid-runtime.mjs'));
+        expect.fail('Config is loaded successfully');
+      } catch (error) {
+        expect(error.message).toMatch(/plugins is not defined/);
+      }
+    }
+  });
+
+  test('handles MODULE_NOT_FOUND properly', async () => {
+    try {
+      await loadConfig(path.join(fixtures, 'module-not-found.js'));
+      expect.fail('Config is loaded successfully');
+    } catch (error) {
+      expect(error.message).toMatch(/Cannot find module 'unknown-module'/);
+    }
+  });
+});