add zim-syntax
Squashed 'bundle/zim-syntax/' content from commit 1638f10

git-subtree-dir: bundle/zim-syntax
git-subtree-split: 1638f104b11a3b79cbcbcbc172674ada5e918713
bundle/zim-syntax: Pulled new 'master' from 'git@github.com:vim-scripts/zim-syntax'
zim ftdetect plugin