Add optional social sharing buttons

- Social share buttons appear at the end of every post and page unless you add share: false to their YAML Front Matter
- Fixes #42

_sass/variables.scss

@@ -37,6 +37,20 @@ $warning			: #dd8338;
 $danger				: #C64537;
 $info				  : #308cbc;
 
+/* brands */
+$facebook-color     : #3b5998;
+$flickr-color       : #ff0084;
+$foursquare-color   : #0cbadf;
+$google-plus-color  : #dd4b39;
+$instagram-color    : #4e433c;
+$linkedin-color     : #4875b4;
+$pinterest-color    : #cb2027;
+$rss-color          : #fa9b39;
+$tumblr-color       : #2c4762;
+$twitter-color      : #55acee;
+$vimeo-color        : #1ab7ea;
+$youtube-color      : #ff3333;
+
 /* links */
 $linkcolor 		    : #343434;
 $link-color       : $linkcolor;