Handle item-list uses of over/back

Now, if the very first paragraph in an over/back sequence is an =item
command, over/back disappear instead of being replaced by a :nested
block. =items thus are correctly numbered.

Note that =items outside an =over/=back structure are unnumbered, which
is equivalent to =item1s that appear in a tompost =over/=back. This can
potentially be used to alert you to errorneous =item commands outside
=over/=back.

Does not currently provide =begin item/=end item for multi-paragraph
items. This currently must be done manually.
Fix =encoding command

Now the encoding actually carries over :P
Add support for semantic headers

Now things like =head1 TITLE will become =TITLE. Mostly for the
synopses, which are written in a weird "POD5 compatible" form of Pod6 at
times.
Fix issue with =TITLE <space> <newline> <title>

It seems to be a perl6 bug, not our bug, but in any case this keeps
trailing spaces off our commands, which is always nice :) .

I think at this point all we have left to tackle are formatting codes,
namely F<> and L<page/sec> (which need to be I<> and L<doc:page#sec>
respectively)