Support org-capture

It would be really cool if ement.el integrated more closely with org-mode's capture mechanism. Specifically, there are 2 workflows I imagine using:

1. Call org-capture from a particular matrix message, so that the added org node includes a link back to that message.
2. Designating a matrix room as a "capture room," where all messages are captured automatically as ement.el receives them. Maybe allow multiple rooms too, one for each capture template you want to use. Would require some scheme to avoid capturing the same message twice, but I think this could be done just by editing the message or adding a reaction emoji, assuming the matrix protocol doesn't have any better feature you can take advantage of.