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 would like to make a mathjax plugin 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 '$'+contents+'$';  }
});
```