Implement 'swap' command.

This patch introduces a new command 'swap' that swaps two containers so
that they assume each other's position and geometry.

fixes #917

include/commands.h

@@ -290,6 +290,12 @@ void cmd_move_scratchpad(I3_CMD);
  */
 void cmd_scratchpad_show(I3_CMD);
 
+/**
+ * Implementation of 'swap [container] [with] id|con_id|mark <arg>'.
+ *
+ */
+void cmd_swap(I3_CMD, const char *mode, const char *arg);
+
 /**
  * Implementation of 'title_format <format>'
  *