Add the ability to disable parallelization

[villfa]

Closes #230

[codecov]

Codecov Report

Merging #233 into master will increase coverage by 1.23%.
The diff coverage is 79.16%.

@@             Coverage Diff              @@
##             master     #233      +/-   ##
============================================
+ Coverage     78.05%   79.28%   +1.23%     
- Complexity      108      115       +7     
============================================
  Files            17       19       +2     
  Lines           319      338      +19     
============================================
+ Hits            249      268      +19     
  Misses           70       70              

Impacted Files	Coverage Δ	Complexity Δ
src/Commands/ChurnCommand.php	0.00% <0.00%> (ø)	6.00 <0.00> (ø)
src/Processes/Handler/ParallelProcessHandler.php	37.50% <50.00%> (ø)	9.00 <1.00> (?)
src/Processes/Handler/SequentialProcessHandler.php	100.00% <100.00%> (ø)	4.00 <4.00> (?)
src/Processes/ProcessHandlerFactory.php	100.00% <100.00%> (ø)	2.00 <2.00> (?)

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