.frankrc for configuration

This pull request is to add support for a configuration file that allows customization of the way the `frank` command line tool behaves. What it does is hopefully explained by my [updated README](https://github.com/mhaylock/Frank/blob/frankrc/README.md).

I've been working on this in isolation and to selfishly meet my own needs, so I would welcome feedback on whether I've taken things in the right direction, and comments on my implementation.

I've tried to document what I've done clearly, but I've possibly gotten a bit too verbose in doing so - would welcome a more concise rewrite of what I've produced if it can convey what is needed.

In my experience this has made it easier to work with a CocoaPods project, and helped address my own preference to avoid putting as many of the Frank files in my project directory as possible (including the frankified target). By removing the "build files" (as I have termed them) from the project directory and keeping them in the gem it also means that Frank's dependencies can be better managed by [bundler](http://gembundler.com/) and removes the need to run `frank update` every time the gem is updated.

I hope others find this helpful in a similar way.