Resolve issue #3 and a whole lot more

This commit adds support for an optional mount point for /dev/sdb:
```
--sdb 25G,/opt/foo
```

This will append `SDB=/opt/foo` to the Kickstart boot line.

This commit also reorganizes the isolinux template directory.

And it adds support for an `--extra` parameter for specifying arbitrary Kickstart boot line options.