i #214 add effort outcome metrics

[carlosparadis]

Add the following outcome metrics:

• file bug frequency: the total number of commits of all bug type issues the file was involved
• file bug churn: the total churn sum of commits of all bug type issues the file was involved
• file non-bug frequency: the total number of commits of all non-bug type issues the file was involved
• file non-bug churn: the total churn sum of commits of all non-bug type issues the file was involved

A similar "file churn" metric function was also added for consistency.

[codecov[bot]]

Codecov Report

Merging #215 (90e1cce) into master (a241585) will decrease coverage by 0.40%. The diff coverage is 0.00%.

:exclamation: Current head 90e1cce differs from pull request most recent head d778173. Consider uploading reports for the commit d778173 to get more accurate results

@@            Coverage Diff            @@
##           master    #215      +/-   ##
=========================================
- Coverage    9.13%   8.73%   -0.40%     
=========================================
  Files          16      17       +1     
  Lines        2179    2278      +99     
=========================================
  Hits          199     199              
- Misses       1980    2079      +99     

Impacted Files	Coverage Δ
R/metric.R	0.00% <0.00%> (ø)
R/parser.R	8.67% <0.00%> (-0.39%)	:arrow_down:
R/text.R	0.00% <0.00%> (ø)