Add support to write coverage text to file

[Nyholm]

Just like for PHPUnit, you may want to store the "coverage text" to a file.

[codecov]

Codecov Report

Merging #609 (43a39e8) into master (069856b) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##              master      #609   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity       606       612    +6     
===========================================
  Files             24        24           
  Lines           1854      1859    +5     
===========================================
+ Hits            1854      1859    +5     

Impacted Files	Coverage Δ	Complexity Δ
src/Runners/PHPUnit/BaseRunner.php	100.00% <100.00%> (ø)	27.00 <0.00> (+1.00)
src/Runners/PHPUnit/Options.php	100.00% <100.00%> (ø)	148.00 <1.00> (+5.00)

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[Slamdunk]

Type make to run the build locally, if you have php installed, so you don't have to wait GA

[Nyholm]

There, finally. =)

I should have checked for Makefiles before I flooded the CI. Anyhow, I believe this PR is ready for review

[Nyholm]

Wohoo. Thank you for merging

[Slamdunk]

Thank you for your contribution