add BDD keywords

Author: Andreas Beer

Robot.tmLanguage

@@ -100,7 +100,14 @@
 <key>name</key>
 <string>keyword.control.robot</string>
 </dict>
-</array>
+<dict>
+<key>comment</key>
+<string>BDD keywords</string>
+<key>match</key>
+<string>(?i)\s+(Given|When|and|but|Then)</string>
+<key>name</key>
+<string>string.robot.bdd</string>
+</dict>	</array>
 <key>scopeName</key>
 <string>source.robot</string>
 <key>uuid</key>