Handle correctly % value in width and height

related to #72
diff --git a/DESCRIPTION b/DESCRIPTION
index 29b0dfb..b665124 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,7 +1,7 @@
 Type: Package
 Package: revealjs
 Title: R Markdown Format for 'reveal.js' Presentations
-Version: 0.9.1.9002
+Version: 0.9.1.9003
 Date: 2017-03-13
 Authors@R: 
     c(person(given = "Hakim",