Add a custom import plugin demo

This is a direct port from the Godot 2.1 version of the demo.

plugins/custom_import_plugin/import_plugin.gd

@@ -0,0 +1,63 @@
+tool
+extends EditorImportPlugin
+
+enum Presets { PRESET_DEFAULT }
+
+func get_importer_name():
+	return "demos.sillymaterial"
+
+func get_visible_name():
+	return "Silly Material"
+
+func get_recognized_extensions():
+	return ["mtxt"]
+
+func get_save_extension():
+	return "res"
+
+func get_resource_type():
+	return "Material"
+
+func get_preset_count():
+	return 1
+
+func get_preset_name(preset):
+	match preset:
+		PRESET_DEFAULT: return "Default"
+		_ : return "Unknown"
+
+func get_import_options(preset):
+	match preset:
+		PRESET_DEFAULT:
+			return [{
+					"name": "use_red_anyway",
+					"default_value": false
+					}]
+		_: return []
+
+func get_option_visibility(option, options):
+	return true
+
+func import(source_file, save_path, options, r_platform_variants, r_gen_files):
+	var file = File.new()
+	var err = file.open(source_file, File.READ)
+	if (err != OK):
+		return err
+
+	var line = file.get_line()
+
+	file.close()
+
+	var channels = line.split(",")
+	if channels.size() != 3:
+		return ERR_PARSE_ERROR
+
+	var color = Color8(int(channels[0]), int(channels[1]), int(channels[2]))
+	var material = SpatialMaterial.new()
+
+	if options.use_red_anyway:
+		color = Color8(255, 0, 0)
+
+	material.albedo_color = color
+
+	return ResourceSaver.save("%s.%s" % [save_path, get_save_extension()], material)

plugins/custom_import_plugin/material_import.gd

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+tool
+extends EditorPlugin
+
+var import_plugin
+
+func _enter_tree():
+	import_plugin = preload("import_plugin.gd").new()
+
+	add_import_plugin(import_plugin)
+
+func _exit_tree():
+	remove_import_plugin(import_plugin)
+	import_plugin = null

plugins/custom_import_plugin/plugin.cfg

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+[plugin]
+
+name="Silly Material Importer"
+description="Imports a 3D Material from an external text file"
+author="George Marques"
+version="1.0"
+script="material_import.gd"

plugins/custom_import_plugin/test.mtxt

@@ -0,0 +1 @@
+0,0,255