WCAG reporting: include a summary of the tests for statistics

As of now, there is close to no statistics concerning the WCAG tests. It makes it difficult to follow the progression of accessibility on XWiki.
There is some information about it in the log, but this information should be stored in an artifact and completed for easier use.

In order to give insightful report we'd need to:
1. Count the total amount of violations, passes and incomplete rules
2. Count for each rule
3. If possible, aggregate the stats at the build level.
4. Generate a file (or file*s*) named `wcagOverview.txt`