Implement --non-billable flag for timetrace start

[dominikbraun]

At the moment, billable records can be created using timetrace start my-project --billable. With PR #166, project records can be billable by default:

# config.yml
projects:
  my-project:
    billable: true

Using this config, timetrace start my-project will produce a billable record.

But if my-project is configured to be billable by default, there also has to be a way to create non-billable records for that project. This should be possible by using a --non-billable flag.