Adding support for optional frontend config.

Dropdown network selector is hidden by default, and enabled using config.
fixes #79

frontend/generate-config.js

@@ -0,0 +1,36 @@
+var fs = require('fs');
+
+const CONFIG_FILE_NAME = 'mempool-frontend-config.json';
+const GENERATED_CONFIG_FILE_NAME = 'generated-config.js';
+
+let settings = [];
+let configContent = {};
+
+try {
+  const rawConfig = fs.readFileSync(CONFIG_FILE_NAME);
+  configContent = JSON.parse(rawConfig);
+} catch (e) {
+  if (e.code !== 'ENOENT') {
+    throw new Error(e);
+  }
+}
+
+for (setting in configContent) {
+  settings.push({
+    key: setting,
+    value: configContent[setting]
+  });
+}
+
+const code = `(function (window) {
+  window.__env = window.__env || {};${settings.reduce((str, obj) => `${str}
+    window.__env.${obj.key} = ${ typeof obj.value === 'string' ? `'${obj.value}'` : obj.value };`, '')}
+  }(this));`;
+
+try {
+  fs.writeFileSync(GENERATED_CONFIG_FILE_NAME, code, 'utf8');
+} catch (e) {
+  throw new Error(e);
+}
+
+console.log('Config file generated');