body() primitive is correct.

  The `body()` primitive will return the same message as set
 by `on_get_body()`, or the message to be displayed as piped
 via the `display_filter()` primitive.

  This closes #127
  Make ::get_body() private.

  This is now an internal-only function, since the `body()`
 lua-primitive no longer needs to call it.
  Added assertion.
  Simplified return-logic.
 Added assertions.
  Added assertions & limits.
make column highlight lua variable

The highlight mode of the current column in maildir/index mode can be
set with maildir/index_highlight_mode(). Available options are
underline,standout,reverse,blink,dim,bold. They map to the corresponding
ncurses video attributes.

This change is useful for people using tmux, which is known for having
trouble with different highlighting modes in ncurses programs.
Merge pull request #132 from kain88-de/highlighting

Good contribution.  THanks.

I'd make a couple of changes - and will fix them up post-merge:

* `src/lua.cc` / `src/variables.cc` - List the new primitives alphabetically
* Add a helper to get the mode from the string.
* Setup a default value for both strings.
  Order definitions alphabetically.
  Order prototypes alphabetically.
  Document function.
  Updated docs for highlight options.

  Added them to the default TAB-completion set too.
  Setup default highlight values.

  Also sort the options.
  Refactor conversion between string and curses attribute.

  Since that cuts down on duplication.