Add generator short cuts

If creating a new page by generator, i have to run
```bash
sipa generate
# or its shortccut "sipa g"
```
and then enter `page` or `p`.

It would be nice, to do that in one step, so valid commands to generate a page in one line woulde be:

```
sipa generate page
sipa g page
sipa g p
sipa generate p
```