Makes the bash hook resistant to double-hooking.

```bash
eval "$(direnv hook bash)"
eval "$(direnv hook bash)"
echo $PROMPT_COMMAND
```
=> _direnv_hook;
Makes the zsh hook resistant to double-hooking.

```zsh
eval "$(direnv hook zsh)"
eval "$(direnv hook zsh)"
echo precmd_functions
```
=> _direnv_hook
Merge pull request #75 from zimbatm/avoid-double-inclusion

Avoid double inclusion