Zsh completion

Copy _tm somewhere in your fpath. I put mine in
`~/.dotfiles/zsh/completions/_tm`.

You'll need to run `autoload -U compinit` and `compinit` after _tm is
added to your fpath in your .zshrc or wherever.
Infer working directory if session name in CDPATH

Automatically set the working directory (the directory windows in a
session begin in) of new sessions to project folders, assuming that they
live in ~/code. The cdpath portion could be removed if cdpath were
already set to something other than `.`.

Using CDPATH necessitated switching the script to zsh instead of sh.
Refactor to use functions, add --help
Add README

* Rename completion.bash
* Incorporate tmux.conf example
Show help text for bare tm outside tmux