Allow non-Dropbox storage location

Relevant to #39.

I keep all my dotfiles under version control [here](https://github.com/jonlong/dotfiles), and have recently been thinking about how nice it would be to keep my app preferences stored alongside them.

Mackup is perfect, but I wanted to specify my own directory (rather than Dropbox), so I modified it to take a new configuration option called "Alternate Storage Location".

@Ira, it's a simplified version of your proposed solution in #39, as I didn't want to get into the nuts and bolts of how to allow for other syncing systems as well (Google Drive, etc).

To use it, check out the additions in the sample config.

I think a nice addition would be implementing @chrishas35's original suggestion of allowing this to be a command line argument as well—when I get some more time, I can look into that, too.

Let me know what you think!