added callback argument to document function

docco.litcoffee

@@ -77,25 +77,37 @@ assets, reading all the source files in, splitting them up into prose+code
 sections, highlighting each file in the appropriate language, and printing them
 out in an HTML template.
 
-    document = (options = {}) ->
+    document = (options = {}, callback) ->
       configure options
 
       exec "mkdir -p #{config.output}", ->
 
-        exec "cp -f #{config.css} #{config.output}"
-        exec "cp -fR #{config.public} #{config.output}" if fs.existsSync config.public
+        steps     = 2
+        completed = 0
+
+        complete  = (error) ->
+          if error
+            if _.isFunction callback then return callback error
+            else throw error
+
+          callback?() if ++completed is steps
+
+        exec "cp -f #{config.css} #{config.output}", complete
+        if fs.existsSync config.public
+          steps++
+          exec "cp -fR #{config.public} #{config.output}", complete
         files = config.sources.slice()
 
         nextFile = ->
           source = files.shift()
           fs.readFile source, (error, buffer) ->
-            throw error if error
+            complete error if error
 
             code = buffer.toString()
             sections = parse source, code
             format source, sections
             write source, sections
-            nextFile() if files.length
+            if files.length then nextFile() else complete()
 
         nextFile()