Add support for assemble block scoped prefixes, and suffixes

[theseion]

The flags, prefix, and suffix comments are very handy but they should be block scoped. That would make reading of blocks. It would also make it much simpler to pass flags, prefixes, and suffixes from an include file parser (see #40).

[theseion]

Implementing this for flags is not possible, unfortunately. See #71 for details.