- added OpenMetrics reporter
- added unit tests for OpenMetrics reporter
- added `openmetrics` to usage help
- updated usage and added description of OpenMetrics to `README.md`
- extracted common language summary aggregation from `toCSVSummary` and `toJSON` into separate method `aggregateLanguageSummary`
- renamed `golang.org/x/text/language` import to avoid warnings about naming conflicts with local variables