feat: emit a started event on consumer start

[nicholasgriffintn]

Resolves #388

Description:

Fires a started event when the Consumer is started, like we do when it is stopped.

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?:

Allows implementations to extend functionality when the event is recieved

Code changes:

• Added the new event
• Emit the event on start

[codeclimate[bot]]

Code Climate has analyzed commit 50dc6ae8 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 94.7% (0.0% change).

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