Thoughts on a few changes

[dstufft]

Hello there!

We're investigating using appdirs inside of pip over in pypa/pip#1733 and a few issues came up. We were wondering if y'all would be interested in any of these changes at all.

1. Make the "vendor" argument optional even on Windows, and simply omit it if it is not provided. While it is true that technically the vendor is a required directory on Windows, it's not a very well followed standard and a lot of things violate it.
2. Ability to override the default values, specifically so we can move the /etc/xdg/<foo> directory to /etc/<foo>. Again while it is true that the XDG spec does mandate defaulting to /etc/xdg looking at a few Linux installs it appears that generally the only things that actually do default to /etc/xdg instead of /etc are actual desktop apps and not CLI programs like pip.

I don't have a problem submitting these pull requests but I wanted to see if these were things that y'all were willing to have!

[srid]

@dstufft +1 - as long as the default behaviour doesn't change. feel free to open a pull request.