Implement parallel runner for concordion:run commands

This addresses concordion/concordion #78. See [README](README.md) for details

It would be useful to increase the coverage of [ParallelRunStrategyTest](src/test/java/org/concordion/internal/command/ParallelRunStrategyTest.java), as per the TODO comments in the file.

Depending on feedback, it may be better to move this into a separate extension rather than being part of Concordion core, since it introduces extra dependencies (Guava and slf4j).