Add parameters to use in the footer

Presentations including the footer should set the LECTURE_TOPIC
LECTURE_NO attributes so that the proper information is displayed
in the footer area of the presentation.

Example:
:LECTURE_TOPIC: Git demo
:LECTURE_NO: 4th lecture

lectures/revealjs/03-dev-and-scm.adoc

@@ -2,6 +2,8 @@
 :customcss: slides.css
 :icons: font
 :includedir: revealjs/includes/
+:LECTURE_TOPIC: Software development
+:LECTURE_NO: 3rd lecture
 
 include::{includedir}header.adoc[]
 

lectures/revealjs/includes/footer.adoc

+[subs="attributes"]
 ++++
  <div id="footer" class="footer">
  	<div style="display:table-row;">
@@ -8,8 +9,8 @@
      				alt="ntnu logo" height="28"/>
      	</a>
      </span>
-     <span class="element" style="width:300px;">| IT1901 </span>
-     <span class="element">| Software Development </span>
+     <span class="element" style="width:300px;">| IT1901 - {LECTURE_NO}</span>
+     	<span class="element">| {LECTURE_TOPIC} </span>
      <span class="element">&nbsp;&nbsp;&nbsp;&nbsp;</span>
   </div>   
  </div>

lectures/revealjs/scm.adoc

@@ -2,6 +2,8 @@
 :customcss: slides.css
 :icons: font
 :includedir: revealjs/includes/
+:LECTURE_TOPIC: Source Code Management
+:LECTURE_NO: 
 
 include::{includedir}header.adoc[]