Improve Markdown (and other) module doc output

- The html_ify filter now escapes HTML found in module documentation.
  THIS COULD AFFECT MORE THAN JUST MARKDOWN but I didn't see any modules
  expecting to use e.g. HTML entities or HTML tags in their
  documentation.

- The markdown_ify filter (used as jpfunc in markdown.j2) escapes at
  least a few Markdown in-line formatting characters.

- Improvements to markdown.j2:

    - Call jpfunc on the module name heading so that it gets escaped for
      Markdown (e.g. `my_module_name` becomes `my\_module\_name`).

    - Added paragraph breaks between paragraphs in the description.

    - Added examples heading, which is consistent with the notes heading
      below it.

I've used a slightly modified version of this markdown.j2 to produce http://dsedivec.github.io/ansible-plugins/.