Finish implementing #98 (encapsulate configuration)

@charlierudolph @kevgo 

* Added ability to run `git town [...]` even before configuring git town.
  * Rationale - the user should be able to see the help text before being immediately prompted by the config prompts
* Added `git town main-branch [<branch_name>]` to view and edit the main branch
* Added `git town non-feature-branches [--add|--remove <branch_name>]` to view and modify non-feature branches

Couple issues:
* the linter on Travis seems to be different from the one I have. Everything passes locally, but Travis' wants me to fix [line 42 of shell_helpers.sh](https://github.com/Originate/git-town/compare/aw-finish-config?expand=1#diff-5fb3dd355b9b26c61663d7e067c373fbR42). Following their suggestion, the functionality seems to change and break.
* The `git town` "option parsing" seems messy -- any advice?
* Don't really like the differences between the flags and "subcommands", e.g. `git town main-branch` vs. `git town --config`. Would `git town config` be better than `git town --config` (per Kevin's original suggestion)?
There's still ambiguity. The last capture group of the first regex will still capture a sentence ending in "(behind|ahead of) main".

```
Given(/^I have(?: a)?( local)?(?: feature)? branch(?:es)? named (.+?)$/) do |local, branch_names|
  Kappamaki.from_sentence(branch_names).each do |branch_name|
    create_branch branch_name, remote: !local
  end
end


Given(/^I have a( local)? feature branch named "(.+?)" (behind|ahead of) main$/) do |local, branch_name, relation|
  create_branch branch_name, remote: !local
  if relation
    commit_to_branch = relation == 'behind' ? 'main' : branch_name
    create_commits branch: commit_to_branch
  end
end
```