Add debug flag to skyux watch that disables code coverage

[Blackbaud-PaulCrowder]

When the istanbul-instrument-loader is added to the Webpack config, the instrumented code becomes unreadable. This is usually fine except for when trying to debug through the original code by navigating to http://localhost:9876/debug.html while skyux watch is running. We should add some kind of command line argument (maybe --debug or --no-coverage) to disable this Webpack loader so that the code becomes readable again and can be stepped through in the browser.