Encapsulate configuration (resolves #98)

`git town --config` will print out the current configuration
`git town --version` shows the Git Town version

Need help with the test for `git town --version`. Unsure how to modify environment variables while testing, so my approach was a temporary workaround.

Todo in future PR:
* `git town` and `git town --help` will show some useful info (TLDR for the man page)
* Way to modify configuration

@charlierudolph @kevgo 
Using `@shell_overrides_export` to store environment variable changes during testing.