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)?
I vote for keeping things simple and redundancy free, and having only one help page here:
* merge these two help pages into the man page for git town
* always show the man page, i.e. show it for
   * `git town`
   * `git town --help`
   * `man git-town`
   * `git help town`

The structure of the man page would be something like:
1. give an overview of Git Town: what it is, what it is for etc
2. list of all the commands it provides
3. how to configure it

I can help polish the wording in it.