Attributes in markdown

# Slide attributes in Markdown
## Allow an external Markdown file to set attributes for each slide   
(replace #639, because the pull request is now rebased on `dev`, not `master`):
````
<div class="slides">
      <section  data-markdown="training.md" 
                data-separator="^\n\n\n" 
                data-vertical="^\n\n" 
                data-notes="^Note:" 
                data-attributes="^\s*?--\s(.*?)$"        <=========
                data-charset="utf-8">
      </section>
      </div> <!-- slides -->
````

For example, that slide will set an id, transition and color:

````
## Plan:
-- id="plan" data-transition="zoom" data-background="#0f0f0f"

My plan is....
````

The slide attributes themselves are *removed* from the final markdown content, and not visible in the resulting html slide.

Incidentally, that should also solve #430 (Markdown Support for internal links);  
Another slide can now refers to "plan" with:

````
[Link to Plan](#/plan)
````