Note that version 1.3 of the vim syntax file has been sent upstream.

Also fix an issue noticed by Bram during integration: Make sure to set "cpo"
correctly. This matches other vim syntax files and is required to make sure
that the backslash continuation works with all .vimrc files
(see `:help line-continuation`).
Merge pull request #547 from nico/vim_cpo

Note that version 1.3 of the vim syntax file has been sent upstream.