Allow markers to be created in a reverse orientation

This means that the head of the marker precedes its tail in the buffer.
This will be important if we want to build selections on top of markers.

src/app/buffer-marker.coffee

@@ -8,13 +8,20 @@ class BufferMarker
   tailPosition: null
   stayValid: false
 
-  constructor: ({@id, @buffer, range, @stayValid, noTail}) ->
-    @setRange(range, {noTail})
+  constructor: ({@id, @buffer, range, @stayValid, noTail, reverse}) ->
+    @setRange(range, {noTail, reverse})
 
   setRange: (range, options={}) ->
     range = @buffer.clipRange(range)
-    @tailPosition = range.start unless options.noTail
-    @headPosition = range.end
+    if options.reverse
+      @tailPosition = range.end unless options.noTail
+      @headPosition = range.start
+    else
+      @tailPosition = range.start unless options.noTail
+      @headPosition = range.end
+
+  isReversed: ->
+    @tailPosition? and @headPosition.isLessThan(@tailPosition)
 
   getRange: ->
     if @tailPosition

src/app/buffer.coffee

@@ -295,6 +295,9 @@ class Buffer
   getMarkerRange: (id) ->
     @validMarkers[id]?.getRange()
 
+  isMarkerReversed: (id) ->
+    @validMarkers[id]?.isReversed()
+
   getAnchors: -> new Array(@anchors...)
 
   addAnchor: (options) ->