Make watching optional

[hurrymaplelad]

I'd like to use grunt-watchify for development, test, and production builds so I only need to configure 1 task in my Gruntfile and all builds benefit from watchify's shared cache speedup (#6). For test and production builds, I'd like to disable rebundles on file changes so tasks further down the pipeline can modify files for server side consumption without triggering a client bundle.

Disabling watch should also cause the task to fail and grunt to exit with an error code on bundle errors.

Perhaps with a watch option to match the old browserify 1.x option.