Improve the usability of Churn

[villfa]

This change makes it possible to run from outside of a project:

./churn.phar run my_project/src

[codecov]

Codecov Report

Merging #257 (5a38965) into master (266cfe0) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #257   +/-   ##
=========================================
  Coverage     95.55%   95.55%           
  Complexity      157      157           
=========================================
  Files            21       21           
  Lines           405      405           
=========================================
  Hits            387      387           
  Misses           18       18           

Impacted Files	Coverage Δ	Complexity Δ
src/Process/ProcessFactory.php	94.73% <100.00%> (ø)	6.00 <0.00> (ø)

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