add support for slim filters

I use slim filters quite a bit when I'm dealing with complex content
with lots of formatting and links... and it's not supported by the
current version of vim-slim.

I basically copied entire @tpope's implementation from vim-haml, (link in the
references), and thus far it works pretty well. I didn't attempt
anything fancy, just to recognize structures like `markdown:` as a valid
keyword and to avoid highlighting anything nested below it.

References:
- https://github.com/tpope/vim-haml/blob/master/syntax/haml.vim#L62