v0.2.0

Changelog

• 965272b: docs: --detach in the README, the host parameters in the VS Code how-to, and what --detach asks of a host (@diegosz)
• 36aad06: docs: ?theme= stays fixed for the life of a frame; the message carries changes (@diegosz)
• 85ed752: docs: stamp v0.2.0 (@diegosz)
• b1ad3ab: feat: a top-level host channel (?embed=top), ?theme=, and the embed surface in /capabilities (@diegosz)
• dd37f65: feat: check the items inside form fields and markup images at the write (@diegosz)
• 6b42f44: feat: serve --detach, a board that outlives the shell that started it (@diegosz)
• 9a2e8f8: fix: admit mwembed: in an html tab's frame-ancestors (@diegosz)