Support for Cobertura XML report format

[smmccabe]

Updated version of #734

I combined some work I had done originally with Rask's work. The result should be fully compliant with the cobertura format dtd.

The one problem I have not resolved is one that Rask noted that cobertura doesn't seem to support non-class based lines. I am not sure if this should even be a blocking issue, but I will check what some other test runners like Jest have done.

[codecov]

Codecov Report

Merging #812 into master will increase coverage by 0.42%.
The diff coverage is 95.79%.

@@             Coverage Diff              @@
##             master     #812      +/-   ##
============================================
+ Coverage     83.09%   83.51%   +0.42%     
- Complexity     1072     1093      +21     
============================================
  Files            61       62       +1     
  Lines          3448     3567     +119     
============================================
+ Hits           2865     2979     +114     
- Misses          583      588       +5     

Impacted Files	Coverage Δ	Complexity Δ
src/Report/Cobertura.php	95.79% <95.79%> (ø)	21.00 <21.00> (?)

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