Parse tag strings with Markdown

[Dox was just recently updated to parse Markdown in tag descriptions](https://github.com/tj/dox/commit/08995f528c7aa33d23a6f1107f7080263d8b28c7), but it does not (currently) do the same for the `string` property of tag objects. (The `string` property is used in place of `description` for tags which do not include a "type" specifier.) Here's an example usage:

```js
/**
 * This option enforces the consistency of quotation marks used throughout
 * your code. It accepts three values: `true` if you don't want to enforce
 * one particular style but want some consistency, `"single"` if you want to
 * allow only single quotes and `"double"` if you want to allow only double
 * quotes.
 *
 * @deprecated JSHint is moving away from enforcing stylistic concerns. Check
 *             out [the JSCS project](https://github.com/jscs-dev/node-jscs)
 *             for similiar functionality.
 */
```

Note that I've had to special-case the `alias` tag. I think this makes sense, but I'd welcome input from a maintainer.