Allow markers to be created with just a position

When a marker is created with just a position, it only gets a head
position and has no tail position.

src/app/buffer-marker.coffee

@@ -8,16 +8,23 @@ class BufferMarker
   tailPosition: null
   stayValid: false
 
-  constructor: ({@id, @buffer, range, @stayValid}) ->
-    @setRange(range)
+  constructor: ({@id, @buffer, range, @stayValid, noTail}) ->
+    @setRange(range, {noTail})
 
-  setRange: (range) ->
+  setRange: (range, options={}) ->
     range = @buffer.clipRange(range)
-    @tailPosition = range.start
+    @tailPosition = range.start unless options.noTail
     @headPosition = range.end
 
   getRange: ->
-    new Range(@tailPosition, @headPosition)
+    if @tailPosition
+      new Range(@tailPosition, @headPosition)
+    else
+      new Range(@headPosition, @headPosition)
+
+  getHeadPosition: -> @headPosition
+
+  getTailPosition: -> @tailPosition
 
   getStartPosition: ->
     @getRange().start

src/app/buffer.coffee

@@ -271,17 +271,26 @@ class Buffer
   getMarkers: ->
     _.values(@validMarkers)
 
-  markRange: (range, options) ->
-    marker = new BufferMarker(_.extend({
+  markRange: (range, options={}) ->
+    marker = new BufferMarker(_.defaults({
       id: @nextMarkerId++
       buffer: this
-      range: range
+      range
     }, options))
     @validMarkers[marker.id] = marker
     marker.id
 
+  markPosition: (position, options) ->
+    @markRange([position, position], _.defaults({noTail: true}, options))
+
   getMarkerPosition: (id) ->
-    @validMarkers[id]?.getPosition()
+    @getMarkerHeadPosition(id)
+
+  getMarkerHeadPosition: (id) ->
+    @validMarkers[id]?.getHeadPosition()
+
+  getMarkerTailPosition: (id) ->
+    @validMarkers[id]?.getTailPosition()
 
   getMarkerRange: (id) ->
     @validMarkers[id]?.getRange()