Fix multiline string highlighting

Both [the TextMate grammar](https://github.com/textmate/yaml.tmbundle) and this CS package have errors in the highlighting of multiblock strings.

From Atom:

![screen shot 2014-12-06 at 10 03 35 am](https://cloud.githubusercontent.com/assets/64050/5327107/149b222a-7d46-11e4-8c28-f383d1754b96.png)

![screen shot 2014-12-06 at 10 07 19 am](https://cloud.githubusercontent.com/assets/64050/5327108/1fb2963e-7d46-11e4-9bef-3f38b2e2dcc8.png)

The Docs team makes heavy use of YAML for its Jekyll/Pages site. In the first screenshot, they key with a decimal value (`2.0`) is not represented properly; this is because the matcher was looking for a `\w`, which I've changed to `\S`. In the second screenshot, although `#` is technically a comment, multiple `#`s interfere with the fact that it's part of the multistring block. When this content is parsed by YAML, it treats it as part of the string, not as a comment.

This PR modifies the grammar to address those cases.

/cc @kevinsawicki 
/cc @bernars @neverett I promised I'd fix it!