Add padding as keyword

[jvdb]

So that the header can have a field definition that describes padding:

format x
extension x

strings ascii
padding 0xFF
size 1

One thing that needs to be sorted out: How extensive should the padding definition be? There are multiple possibilities:

1. Instead of a definition, refer to a padding structure defined in the structures section.
2. A complete field definition, such as padding: 0xFF size 1; but then the : and ; syntax looks out of place. Maybe this can be omitted.
3. Just a single value using the format's defaults as shown in the example above. This may be too limited to effectively express different variants of padding encountered.