Allow dollar sign for inline dialects

With this change it is possible to use the dollar ($) sign for inline dialects. I have tested this using vagrant. 

I have made a [mathjax plugin](http://meta.discourse.org/t/brand-new-plugin-interface/8793/61) that escapes everything between $...$ signs from the markdown parser, and I need this to let the following code work:

```
Discourse.Dialect.inlineBetween({
  start: '$',
  stop: '$',
  rawContents: true,
  emitter: function(contents) { return '$'+contents+'$';  }
});
```

[This](http://meta.discourse.org/t/problem-with-dollar-sign-in-custom-markdown-dialect/11065/3) the related topic at meta.discourse.org.