add cli option to not use workers (for dynamic content compatibility)

This adds a `--no-workers` option for `roots watch` and `roots compile` that forces roots to use one process instead of distributing across workers. This is for compatibility with the [dynamic-content](https://github.com/carrot/roots-dynamic-content) extension. Currently with workers enabled, multiple dynamic content files from one category are split between several workers. That means any workers who have to compile a view that lists all dynamic content in a category will be missing large amounts of content in the list because not all the files make it into the same worker.