Added new test/demo-program.

    ./dump-parts ~/Maildir/.steve.org.uk/cur/1178920480.000011.mbox:2,RS
    File: /home/skx/Maildir/.steve.org.uk/cur/1178920480.000011.mbox:2,RS
    1 text/plain
    2 image/gif
    3 image/jpeg
    4 image/jpeg

  This is iterating over the body of the named message, and showing
 the multipart contents.

  We're intending to write three(??) primitives which will be similar
 to :

* get_body_part( N ) - Returns the named part.
* get_body_parts() - Which returns a table similar to the output above.
* has_body_part( "text/plain" ) - Returns a boolean.

  Somehow we'll wire things up so that a function can return text
 for body display.

  This is part of 114.