Allow arbitrary headers

Squashed commit of the following:

commit 3b9156478628474e1abe92e8d543c4496ca597d6
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Wed Apr 17 21:39:35 2013 +0100

    Update (admittedly sparse) documentation

commit a610c8f444eb5a97e3596f987dcf554cdc395ee5
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Wed Apr 17 21:34:03 2013 +0100

    Use custom macro name

    Consistently hide Groff from the user.

commit c239c2af28c6e9c201cb6168072e100dcefd228e
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Wed Apr 17 20:40:05 2013 +0100

    Copy across changes

    Copy custom-header-enabling changes from movie macro set to manga
    macro set.

commit c4d430cd61ef1299fd7cfc4dce1a0e852015d16d
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Wed Apr 17 20:13:53 2013 +0100

    Fix bug, allowing arbitrary headers

    I was going to ask for help on the Groff mailing list, then realised
    I already did:
    http://lists.gnu.org/archive/html/groff/2012-06/msg00013.html By an
    astonishing coincidence, this previous e-mail of mine and its first
    reply present and fix the exact same problem.

commit 36bf4bb8445a3832192adc02830823b23cae9fa4
Author: Zoë Blade <zoe@bytenoise.co.uk>
Date:   Wed Apr 17 00:52:09 2013 +0100

    Allow arbitrary headers

    This doesn't seem to work yet.  For some reason, I can't get defined
    strings (.ds) to echo out within title lines (.tl).