Add config option for watch polling.

[LinusBorg]

There seem to be quite a lot of environments (e.g. dropbox folders, docker containers, vagrant ...) in which webpack-dev-middleware's default mode for file watching doesn't work correctly, and thus HMR fails.

In those scenarios, switching to a polling mode seems to fix most people's problems.

This PR adds a new config option for the dev environment to switch to poll mode - this adds the following options to webpack-dev-middleware:

watchOptions: {
  aggregateTimeout: 300,
  poll: true
},

[LinusBorg]

This will be done in #975