have `addWatchTarget` and module cache busting only run when `--watch` configuration is set

In #11 , cache busting was added to support live reloading of modules in NodeJS.  Ideally, this should only be enabled when in `--watch` mode just to avoid unnecessary overhead, specifically
1. [ ] `addWatchTarget`
1. [ ] `eleventy.beforeWatch`

Should probably add something to README too?