title: Force dask to_parquet to write single file
body: <p>When using <code>dask.to_parquet(df, filename)</code> a subfolder <code>filename</code> is created and several files are written to that folder, whereas <code>pandas.to_parquet(df, filename)</code> writes exactly one file.
  Can I use dask's <code>to_parquet</code> (without using <code>compute()</code> to create a pandas df) to just write a single file?</p>