Add support for importing and flushing all models

[grantholle]

This PR adds the ability to flush and import all models configured in a scout config. This is helpful to run a single command to handle all your model updates, such as during deployments when you may have adjusted model settings and need to have them re-indexed.

It checks to see if you have included the now optional model argument or new --all flag, falling back to the argument. If neither are provided, a RuntimeException is thrown alerting the user to the missing model argument.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!