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.