Check if user is in a repo before `feature list` starts.

This prevents `feature list` from listing a bunch of error messages.
Check if in a repo before all feature/hotfix commands.

Don't check if asking for --help.
Correct help flag logic.
Merge check-for-git-repo-for-feature-list (#82) into master

Check if user is in a repo before `feature list` starts.

This prevents `feature list` from listing a bunch of error messages:

```
rchoudhury@cominor ~
> feature list
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

Current Branch:
------------------------------------------------------------
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
(not on any branch!)

Available feature branches:
------------------------------------------------------------
(none)
fatal: Not a git repository (or any parent up to mount point /mnt/ebs)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
/mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `initialize': Permission denied - /git-scripts.log (Errno::EACCES)
        from /mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `open'
        from /mnt/ebs/home/ifixit/git-scripts/lib/helpers.rb:174:in `log_command'
        from /mnt/ebs/bin/feature:260:in `<main>'
```