add wx-open-launch-app support.

[yanglb]

support wx-open-launch-app event.

Wechat.isInstalled((installed) => {
  if (!installed) return
  document.addEventListener(
    'wechat.launchFromWX',
    ({ extinfo }) => {
      console.log('launch from wechat extinfo = ', extinfo)
    },
    false
  )
})

[xu-li]

多谢提交。我就直接merge了。