Fix gtfo with one CLI argument

```
⊙ gtfo nyc Julian@home
Traceback (most recent call last):
File "/Users/Julian/.local/bin/gtfo", line 11, in
load_entry_point('gtfo==0.3.0.post4', 'console_scripts', 'gtfo')()
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/click/core.py", line 722, in __call__
return self.main(*args, **kwargs)
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/click/core.py", line 697, in main
rv = self.invoke(ctx)
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/click/core.py", line 895, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/click/core.py", line 535, in invoke
return callback(*args, **kwargs)
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/gtfo/_cli.py", line 30, in main
search = _roundtrip(destinations=destinations)
File "/Users/Julian/.local/share/virtualenvs/gtfo/site-packages/gtfo/_cli.py", line 14, in _roundtrip
departure, arrival = destinations
ValueError: expected length 2, got 1

```