Adding Slack.com theme

The newly minted Slack.com has some bright new colors. Here is a theme based on them!

data/themes.yml

@@ -208,3 +208,6 @@
 -
   :name: 'Douglas Rain'
   :colors: '#EDECEB,#E3E3E3,#DA552F,#FFFFFF,#D6D6D6,#534540,#DA552F,#DA552F'
+- 
+  :name: 'Slack.com'
+  :colors: '#573D82,#453068,#3FBA91,#FFFFFF,#453068,#FFFFFF,#3FBA91,#FF3E88'