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 
Hard-coding the spec to the current version like this will break when we release a new version. Your spec should set the version of Git Town somehow, i.e. change it wherever you read it from.

```cucumber
Given the current Git Town version is "0.4.1"
When I run `git town --version`
Then I see "Git Town 0.4.1"
```