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.