feature: add nightmare.cookies.flush()

[matthewmueller]

Based on our discussion in #1296, it'd be good to support: await nightmare.cookies.flush().

That would involve adding the action to action.js along with some tests. Here's the custom action it would be based on:

// try to flush the session data
function CookieFlusher(Nightmare) {
  Nightmare.action(
    'flushCookies',
    (name, options, parent, win, renderer, done) => {
      parent.respondTo('flushCookies', done => {
        win.webContents.session.cookies.flushStore(done)
      })
      done()
    },
    function(done) {
      this.child.call('flushCookies', done)
    }
  )
}