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 
FYI it is more robust to use the non-greedy form of regex in step definitions: "(.+?)" instead of "(.+)". The reason is that the latter form is greedy, i.e. tries to grab as many characters as it can. If you have a step definition __I have set "foo" and "bar" and "baz" to "hello"__, the greedy regex in your current step definition would match __foo" and "bar" and "baz__