Add JSON/YAML components (#267)

Add CodeMirrorInput and VariablesField

Add components for syntax highlighting, YAML/JSON toggle

src/util/yaml.js

@@ -0,0 +1,17 @@
+import yaml from 'js-yaml';
+
+export function yamlToJson (yamlString) {
+  const value = yaml.safeLoad(yamlString);
+  if (!value) { return '{}'; }
+  if (typeof value !== 'object') {
+    throw new Error('yaml is not in object format');
+  }
+  return JSON.stringify(value, null, 2);
+}
+
+export function jsonToYaml (jsonString) {
+  if (jsonString.trim() === '') { return '---\n'; }
+  const value = JSON.parse(jsonString);
+  if (Object.entries(value).length === 0) { return '---\n'; }
+  return yaml.safeDump(value);
+}