hit crashes when ~/.hashdist/config.yaml is not present

```
~/s/spec git:spec ❯❯❯ python bin/hit help show                                                                                                           ⏎
[ERROR] Uncaught exception:
Traceback (most recent call last):
  File "/Users/aron/sandbox/spec/hashdist/cli/main.py", line 147, in help_on_exceptions
    return func(*args, **kw)
  File "/Users/aron/sandbox/spec/hashdist/cli/main.py", line 121, in main
    config = load_config_file(args.config_file)
  File "/Users/aron/sandbox/spec/hashdist/formats/config.py", line 59, in load_config_file
    doc = load_yaml_from_file(filename)
  File "/Users/aron/sandbox/spec/hashdist/formats/marked_yaml.py", line 191, in load_yaml_from_file
    with open(filename) as f:
IOError: [Errno 2] No such file or directory: '/Users/aron/.hashdist/config.yaml'

This exception has not been translated to a human-friendly error message,
please file an issue at https://github.com/hashdist/hashdist/issues pasting
this stack trace.
```

Why does the `hit` tool rely on config.yaml being present to work?  This issue is also causing one of the tests (that relies on `hit` working correctly) to fail on my Mac.