Breaks parsing @media rules

[CTaylor1118]

The parsing for consecutive @media rules (and likely @Keyframes and @font-face) is broken. It seems to be due to the code assuming that @ is the next character (https://github.com/mozilla/slowparse/blob/gh-pages/slowparse.js#L224) when it may be a space. I can make a PR if needed.

[Pomax]

Thanks for letting us know - a PR is always appreciated!