Plumb status updates through event handler

[cottsay]

This will allow users to disable the event handler and silence the currently unconditional console output.

Closes #37

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 21.05263% with 15 lines in your changes missing coverage. Please review.

Project coverage is 73.33%. Comparing base (f52f1b4) to head (1efb41b).

Files with missing lines	Patch %	Lines
...parallel_executor/event_handler/parallel_status.py	0.00%	13 Missing ⚠️
colcon_parallel_executor/event/executor.py	75.00%	1 Missing ⚠️
colcon_parallel_executor/executor/parallel.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
- Coverage   80.30%   73.33%   -6.97%     
==========================================
  Files           2        4       +2     
  Lines         132      150      +18     
  Branches       40       43       +3     
==========================================
+ Hits          106      110       +4     
- Misses         16       30      +14     
  Partials       10       10              

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[tfoote]

Lgtm