README.md: explicitly link to wiki for docs

sh/fish: add d, d. and d.. shortcuts

Normalize config folder name

When I decided the project would be called "dirp",
I didn't align the XDG related path to the config file

dirp hook: dash compatibility

This sacrifices one-line ZSH installability
on some systems.

dirp hook: ZSH compat

Turns out $status is a special
reserved var in ZSH.

After using a different name, the
existing hook is tested as compatible.

Breaking: shell hook defaults

Fish is not the default shell anymore. For traditional
shells, users now need to call:

```sh
eval "$(dirp hook)"
```

Fish users should use:

```fish
dirp hook fish | source
```

Previously, users would call `dirp hook bash`. Now
the shell hook is compatible with BusyBox and a few
other variants, so the default has been changed.

cleanup var

port "dir cfg" to bash

 * exit 2 from `dirp` signals that stdout contains path
   to config file

cleanup var