Message: Add forward method #68

There's no way to relay a message from one room to another. This is useful for bots that mirror announcements across rooms.

Proposed API

async def forward(self, room: Room) -> Message

Example

target = bot.get_room("!other123:matrix.org")
await ctx.message.forward(target)

Before opening a PR

• Write unit tests
• Run mypy matrix/
• Run pytest tests/
• Run black .