ENH add --global-config and --ignore-local-config arguments

fixes #129.

EDIT: behaviour changed, see [comments below](https://github.com/hhatto/autopep8/pull/167#issuecomment-56284739).

~~By default autopep8 looks up config files:~~

- ~~First locally (in current dir) then the parent dir etc. if not found it uses from the DEFAULT_CONFIG file.~~
- ~~If you pass it a specific file it just uses that.~~
- ~~If you pass False, i.e. something which isn't a file, it just uses the standard defaults.~~
- ~~Any arguments you pass in overide defaults/configs.~~

Note: When I say "uses", I mean that values are updated over the standard defaults.

It's straightforward to tweak this behaviour (Note: this implementation is *basically* straight out of [pep8radius](https://github.com/hayd/pep8radius), although that atm does something slightly different with regard to which files to use).

I think this solution is the easiest/cleanest to implement so just did it so it can be discussed/changed.
Actually `--global-config-wrong` will be caught in the next stage (when the arguments are actually parsed).

I had not realised that even `--g foo` works... is this too inelegant for you?
```
    for i, arg in enumerate(arguments):
        if arg.startswith('--g'):
            if '=' in arg:
                config_file = arg.split('=', 1)[1]
            elif i + 1 >= len(arguments):
                config_file = arguments[i + 1]
            else:
                config_file = ''
            return os.path.expanduser(config_file)
```