async/await are not highlighted as keywords

`async` and `await` were introduced as keywords in LiveScript 1.6.0, but this module's syntax stuff doesn't highlight them yet. I guess they'd go in [this list](https://github.com/gkz/vim-ls/blob/795568338ecdc5d8059db2eb84c7f0de3388bae3/syntax/ls.vim#L35)?