Accept integers and other types as call arguments

[meisterluk]

Assume the following command:

  doctl compute droplet neighbors 42 --output json

Because in a CLI, everything is considered as a string, this call is just fine.
However, assume the following call in doctl:

  doctl.compute.droplet.neighbors(42)

This one will fail with the following error:

Traceback (most recent call last):
  File "main.py", line 114, in upgrade
    print(doctl.compute.droplet.neighbors(droplet['id']))
  File "/home/user/venv/lib/python3.6/site-packages/doctl.py", line 224, in neighbors
    return self.do.doctl("compute", "droplet", "neighbors", droplet_id)
  File "/home/user/venv/lib/python3.6/site-packages/doctl.py", line 82, in doctl
    args = " ".join(args)
TypeError: sequence item 3: expected str instance, int found

Apparently, 42 must be a string in order to work out. str(42) seems non-intuitive and therefore I propose this change.