Define base variables before running a extension

Almost each hubctl extensions define the same base environment variables.

```bash
HUB_HOME="${HUB_HOME:-$(dirname "$0")}"
HUB_WORKDIR="${HUB_WORKDIR:-$(pwd)}"
HUB_EXTENSION="$(basename "$0" | sed 's/-/ /g')"
```

Where:

* `HUB_HOME` is a path to the root directory of hubctl extensions
* `HUB_WORKDIR` is a path where hubctl is running
* `HUB_EXTENSION` is the user-friendly name of the hubctl extension

And adds them to `PATH`: `PATH="$HUB_HOME:$HUB_HOME/bin:$HUB_WORKDIR:$HUB_WORKDIR/bin:$PATH"`

Such an approach leads to inconsistency in the development of new extensions where the exact definition is added by copy/paste or even skipped.

We could improve this situation by adding these variables before running the extension.

- [ ] Add base variables before running the extension
- [ ] Review and cleanup variables in existing extensions
- [ ] Document these variables