Support opening non-existent files from the CLI

This required changing text-buffer to support having a
path but not underlying file that exists yet.

Now calling RootView.open() with a non-existed path will
open a dirty empty editor to the path and the file will be
created on first save.

src/app/atom.coffee

@@ -213,7 +213,7 @@ _.extend atom,
         @pendingBrowserProcessCallbacks[messageId]?[callbackIndex]?(data...)
       when 'openPath'
         path = data[0]
-        rootView?.open(path) if fsUtils.isFile(path)
+        rootView?.open(path)
 
   setWindowState: (keyPath, value) ->
     windowState = @getWindowState()

src/app/text-buffer.coffee

@@ -37,13 +37,14 @@ class Buffer
     @lineEndings = []
 
     if path
-      throw "Path '#{path}' does not exist" unless fsUtils.exists(path)
       @setPath(path)
       if initialText?
         @setText(initialText)
         @updateCachedDiskContents()
-      else
+      else if fsUtils.exists(path)
         @reload()
+      else
+        @setText('')
     else
       @setText(initialText ? '')