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)?
```bash
function value_or_none {
  if [ -z "$1" ]; then
    echo '[none]'
  else 
    echo "$1"
  fi
}

function show_main_branch {
  echo "Main branch: $(value_or_none "$main_branch_name")"
}

function show_non_feature_branches {
  echo "Non-feature branches: $(value_or_none "$non_feature_branch_names")"
}