Add GUT (CLI only) and remove gd-plug

Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.

Modify GUT to be used by command-line only.

For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```

addons/gut/UserFileViewer.gd

@@ -0,0 +1,66 @@
+extends WindowDialog
+
+onready var rtl = $TextDisplay/RichTextLabel
+var _has_opened_file = false
+
+
+func _get_file_as_text(path):
+	var to_return = null
+	var f = File.new()
+	var result = f.open(path, f.READ)
+	if result == OK:
+		to_return = f.get_as_text()
+		f.close()
+	else:
+		to_return = str("ERROR:  Could not open file.  Error code ", result)
+	return to_return
+
+
+func _ready():
+	rtl.clear()
+
+
+func _on_OpenFile_pressed():
+	$FileDialog.popup_centered()
+
+
+func _on_FileDialog_file_selected(path):
+	show_file(path)
+
+
+func _on_Close_pressed():
+	self.hide()
+
+
+func show_file(path):
+	var text = _get_file_as_text(path)
+	if text == "":
+		text = "<Empty File>"
+	rtl.set_text(text)
+	self.window_title = path
+
+
+func show_open():
+	self.popup_centered()
+	$FileDialog.popup_centered()
+
+
+func _on_FileDialog_popup_hide():
+	if rtl.text.length() == 0:
+		self.hide()
+
+
+func get_rich_text_label():
+	return $TextDisplay/RichTextLabel
+
+
+func _on_Home_pressed():
+	rtl.scroll_to_line(0)
+
+
+func _on_End_pressed():
+	rtl.scroll_to_line(rtl.get_line_count() - 1)
+
+
+func _on_Copy_pressed():
+	OS.clipboard = rtl.text