added `lineNumbers` option

Over in autodoc[https://github.com/dtao/autodoc], I am generating test code based on JsDoc comments, which are parsed w/ doctrine. It would be useful (for reporting helpful error messages) to be able to indicate where in the source this generated code comes from. So I thought it made sense to add this as an option to doctrine.

This PR adds support for a `lineNumbers` option to add a `lineNumber` property to each tag indicating where that tag is defined within the comment.