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)?
Having difficulty not repeating myself here. It's really already covered by the tagline "general Git Town help, view and change Git Town configuration".

Don't think this is necessary, unless you can think of something to put here.