add option to use aria2c

[gngdb]

I hit an error I wasn't able to debug trying to use the downloader:

Traceback (most recent call last):
  File "/h/gngdb/miniconda3/lib/python3.7/threading.py", line 926, in
_bootstrap_inner
    self.run()
  File "/h/gngdb/miniconda3/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/h/gngdb/miniconda3/lib/python3.7/multiprocessing/pool.py", line
470, in _handle_results
    task = get()
  File "/h/gngdb/miniconda3/lib/python3.7/multiprocessing/connection.py",
line 251, in recv
    return _ForkingPickler.loads(buf.getbuffer())
TypeError: __init__() missing 1 required positional argument: 'content'

Instead of fixing it I added an option to use aria2c (if available). I've found it to be more reliable and it can continue incomplete downloads. As far as I know, the files downloaded are exactly the same.

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