IE9 compatability

I don't know about other versions, but my version of IE9 doesn't support string characters using array indices.

Replaced "text[...]" with "text.charAt(...)" which fixed the issue for me.
Merge pull request #86 from geraintluff/master

IE9 doesn't support addressing characters as array indices
Add JSHint support.

Use `npm run check` or `npm run lint`.
Fix a few JSHint warnings.
.jshint.rc: Enable curly rule.
Reindent lib/markdown.js.