Improve filters and directives documentation rendering.

Hi there!

I'm new with Docular, and I find it great! The output documentation is really nice, and it's now a pleasure to document my directives, filters and services!

I've changed a few things to improve the rendering for filters and directives.

* **Directives**: I think it's better to use the directive's `id` (instead of its `shortName`) to generate the sample code. This allows a human readable name (within `@name`) while still having a correct example. (Human readable names really look nicer in the directives list in the left column!)
* **Filters**: I think it's better to use the first parameter's name (if any) as the input expression. It makes more sense :-)

Can you have a look at this pull request and give me feedback?

Thank you very much for your answer and for this great project!