[vim plugin] 'External' keyword missing

[Valentin MARTINET]

Hi all,

It would be great if you could upgrade the lisaac.vim file
by adding the keword 'External' in the keywords definition
(/editor/vim/syntax/lisaac.vim, l.23).

It was looking bad in my vim when I wrote 'Section External', it was
not displayed as if I wrote 'Section Header/Inherit/...'.

Thanks!

Valentin