Add full example plugin

Also switch to yaml plugin metadata in example file (ref #33)

example-plugin/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2018 Tulir Asokan
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

example-plugin/build.sh

@@ -0,0 +1,2 @@
+#!/bin/bash
+zip -9r helloworld.mbp  maubot.yaml helloworld.py

example-plugin/helloworld.py

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+from maubot import Plugin, MessageEvent
+from mautrix.types import EventType
+
+
+class HelloWorldBot(Plugin):
+    async def start(self) -> None:
+        self.client.add_event_handler(self.handler, EventType.ROOM_MESSAGE)
+
+    async def stop(self) -> None:
+        self.client.remove_event_handler(self.handler, EventType.ROOM_MESSAGE)
+
+    async def handler(self, event: MessageEvent) -> None:
+        if event.sender != self.client.mxid:
+            await event.reply("Hello, World!")

example-plugin/maubot.yaml

@@ -1,19 +1,37 @@
 # This is an example maubot plugin definition file.
-# All plugins must include a file like this named "maubot.ini" in their root directory.
-[maubot]
-# The unique ID for the plugin. Java package naming style.
-ID = xyz.maubot.plugin
+# All plugins must include a file like this named "maubot.yaml" in their root directory.
+
+# The unique ID for the plugin. Java package naming style. (i.e. use your own domain, not xyz.maubot)
+id: xyz.maubot.example
+
 # A PEP 440 compliant version string.
-Version = 1.0.0
+version: 1.0.0
+
 # The SPDX license identifier for the plugin. https://spdx.org/licenses/
 # Optional, assumes all rights reserved if omitted.
-License = AGPL-3.0-or-later
-# The comma-separated list of modules to load from the plugin archive.
+license: MIT
+
+# The list of modules to load from the plugin archive.
+# Modules can be directories with an __init__.py file or simply python files.
 # Submodules that are imported by modules listed here don't need to be listed separately.
 # However, top-level modules must always be listed even if they're imported by other modules.
-Modules = plugin
+modules:
+- helloworld
+
 # The main class of the plugin. Format: module/Class
 # If `module` is omitted, will default to last module specified in the module list.
 # Even if `module` is not omitted here, it must be included in the modules list.
 # The main class must extend maubot.Plugin
-MainClass = PluginClass
+main_class: HelloWorldBot
+
+#  Extra files that the upcoming build tool should include in the mbp file.
+#extra_files:
+#- base-config.yaml
+#- LICENSE
+
+# List of dependencies
+#dependencies:
+#- foo
+
+#soft_dependencies:
+#- bar>=0.1