Implement loadConfig utility (#1328)

Ref https://github.com/svg/svgo/issues/1327

Config file now can only be js. `svgo.config.js` is searched by default.
Otherwise any js module specified in `--config` cli flag.

Config loader is exposed in entry point as `loadConfig(configFile, cwd)`.

lib/svgo-node.js

@@ -0,0 +1,54 @@
+'use strict';
+
+const fs = require('fs');
+const path = require('path');
+const {
+  extendDefaultPlugins,
+  optimize,
+  createContentItem,
+} = require('./svgo.js');
+
+const importConfig = async configFile => {
+  try {
+    const config = require(configFile);
+    if (config == null || typeof config !== 'object' || Array.isArray(config)) {
+      throw Error(`Invalid config file "${configFile}"`);
+    }
+    return config;
+  } catch (error) {
+    if (error.code === 'MODULE_NOT_FOUND') {
+      return null;
+    }
+    throw error;
+  }
+};
+
+const loadConfig = async (configFile, cwd = process.cwd()) => {
+  if (configFile != null) {
+    if (path.isAbsolute(configFile)) {
+      return await importConfig(configFile);
+    } else {
+      return await importConfig(path.join(cwd, configFile));
+    }
+  }
+  let dir = cwd;
+  while (true) {
+    try {
+      const file = path.join(dir, "svgo.config.js");
+      const stats = await fs.promises.stat(file);
+      if (stats.isFile()) {
+        return await importConfig(file);
+      }
+    } catch {}
+    const parent = path.dirname(dir);
+    if (dir === parent) {
+      return null;
+    }
+    dir = parent;
+  }
+};
+
+exports.loadConfig = loadConfig;
+exports.extendDefaultPlugins = extendDefaultPlugins;
+exports.optimize = optimize;
+exports.createContentItem = createContentItem;