WIP: (#10) Add filtering of alloc notifies by state

This PR adds the ability to filter notifications for allocation events by the ClientStatus of the allocation.

This is a work in progress, but I'm opening the PR to gather feedback.

The current implementation is quite rudimentary:

• Simple CLI flag for whitelist/blacklist of states
• no verification if the state provided over the CLI is a valid state

This also hasn't been tested (yet).

Thanks @jrasell, I've taken your comments on board for the most part, except one with trespect to adding a new filtering file because I wasn't sure what you meant.

If you can clarify that I can update the PR and then it should be ready.

Do you want me to combine my commits into one (rebase) before you merge? I'm not sure if you prefer keeping individual commits or squashing them