message: add toMatchKeys() matcher for object subsets
diff: @@ -16,6 +16,13 @@ const stringify = d => JSON.stringify(d, null, ' ')
  const ALIAS_PROP_TYPES = ['w', 'align', 'justify', 'wrap']
  expect.extend({
  + toMatchKeys(obj, values) {
  + return {
  + pass: Object.keys(values).every(key => this.equals(obj[key], values[key])),
  + message: () => `Expected ${stringify(obj)} to have matching keys: ${stringify(values)}`
  + }
  + },
  +
  toHaveClass(node, klass) {
  const classes = getClasses(node)
  const pass = classes.includes(klass)
  