Start new session if none with basename exists.

When using `tat` to connect to a tmux session, I want to start a new
session if the folder name does not already have its own tmux session.
Personal files take precedence over stock files.

The folder order in Thoughtbot's rcrc file results in group files
clobbering personal ones. For cases where I want to override bin files
or zsh functions, use mine as the primary source.