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/diff_formatter.gd

@@ -0,0 +1,67 @@
+var _utils = load("res://addons/gut/utils.gd").get_instance()
+var _strutils = _utils.Strutils.new()
+const INDENT = "    "
+var _max_to_display = 30
+const ABSOLUTE_MAX_DISPLAYED = 10000
+const UNLIMITED = -1
+
+
+func _single_diff(diff, depth = 0):
+	var to_return = ""
+	var brackets = diff.get_brackets()
+
+	if brackets != null and !diff.are_equal:
+		to_return = ""
+		to_return += str(
+			brackets.open,
+			"\n",
+			_strutils.indent_text(differences_to_s(diff.differences, depth), depth + 1, INDENT),
+			"\n",
+			brackets.close
+		)
+	else:
+		to_return = str(diff)
+
+	return to_return
+
+
+func make_it(diff):
+	var to_return = ""
+	if diff.are_equal:
+		to_return = diff.summary
+	else:
+		if _max_to_display == ABSOLUTE_MAX_DISPLAYED:
+			to_return = str(diff.get_value_1(), " != ", diff.get_value_2())
+		else:
+			to_return = diff.get_short_summary()
+		to_return += str("\n", _strutils.indent_text(_single_diff(diff, 0), 1, "  "))
+	return to_return
+
+
+func differences_to_s(differences, depth = 0):
+	var to_return = ""
+	var keys = differences.keys()
+	keys.sort()
+	var limit = min(_max_to_display, differences.size())
+
+	for i in range(limit):
+		var key = keys[i]
+		to_return += str(key, ":  ", _single_diff(differences[key], depth))
+
+		if i != limit - 1:
+			to_return += "\n"
+
+	if differences.size() > _max_to_display:
+		to_return += str("\n\n... ", differences.size() - _max_to_display, " more.")
+
+	return to_return
+
+
+func get_max_to_display():
+	return _max_to_display
+
+
+func set_max_to_display(max_to_display):
+	_max_to_display = max_to_display
+	if _max_to_display == UNLIMITED:
+		_max_to_display = ABSOLUTE_MAX_DISPLAYED