Update logic and auto-complete settings

- If session already exists, attach to it.
- If not, create a new session and try to jump into a matching
  code directory (and try to run a .tmux file to set up the
  session).
- Use both existing session names and code directory names for
  auto-completion.
- Added comments throughout.
Merge pull request #1 from jgeewax/patch-1

Update logic and auto-complete settings