v1.9

Mostly bug fixes (transparent gifs not always showing), but also UI improvements in key strokes (= to set preview size, w to fit width, f for fit screen / fast preview (capital for always, lowercase for once). Clarify design goals. Can now suppress the "Please install X to get Y" messages by setting shownotices="". Fix sixel not detect if the "4" in the terminal's response is first or last.