Add greyscale image exporting

.gitignore

@@ -1,3 +1,4 @@
 .DS_Store
 node_modules
 index.html
+output.png

lib/psd/image.coffee

@@ -8,6 +8,7 @@ module.exports = class Image extends Module
   @includes ImageFormat.RAW
   @includes ImageFormat.RLE
   @includes ImageMode.RGB
+  @includes ImageMode.Greyscale
   @includes Export.PNG
 
   COMPRESSIONS = [
@@ -38,6 +39,7 @@ module.exports = class Image extends Module
 
   setChannelsInfo: ->
     switch @mode()
+      when 1 then @setGreyscaleChannels()
       when 3 then @setRgbChannels()
 
   calculateLength: ->

lib/psd/image_mode.coffee

@@ -1,2 +1,3 @@
 module.exports =
-  RGB: require('./image_modes/rgb.coffee')
+  RGB: require('./image_modes/rgb.coffee')
+  Greyscale: require('./image_modes/greyscale.coffee')

lib/psd/image_modes/greyscale.coffee

@@ -0,0 +1,14 @@
+module.exports =
+  setGreyscaleChannels: ->
+    @channelsInfo = [{id: 0}]
+    @channelsInfo.push {id: -1} if @channels() is 2
+
+  combineGreyscaleChannel: ->
+    for i in [0...@numPixels]
+      grey = @channelData[i]
+      alpha = if @channels() is 2
+        @channelData[@channelLength + i]
+      else
+        255
+
+      @pixelData.push grey, grey, grey, alpha