To improve startup time (#78) we could provide a framework where source files are optimistically transpiled and cached in the main process. No require hooks would be required in the test files avoiding the Babel startup penalty. Thanks @tomazzaman for the suggestion.

• --require hooks won't do, we should support plugins and wrap them like how caching-precompiler wraps Babel
• The watcher needs to recompile changed source files before tests are rerun
• The test worker won't know which cache misses should have been precompiled, so tests may blow up. This would probably be due to insufficient --source patterns so would require useful error messaging somehow