Metrics for plugins other than `warn`

[petertrr]

For warn-plugin we calculate and store count of matched, expected and other warnings. Based on this data we can calculate various scores and use them in save-cloud.

For fix-plugin a possible idea for a metric is number of lines in the diff between expected file and actually fixed file.

For fix-and-warn-plugin we'll need a way to combine those metrics.

Maybe, a separate data class could be introduced for all these counts.