convert ansi escape codes to html server-side

this is much faster than any JS-based solution. the convertor is careful to:

* respect utf8 code points, as the message is text
* group escape code sequences that cancel out so there aren't millions
  of DOM elements
* send self-contained batch messages with no open tag leaking through the next

also switch go go.net/websocket which has a much more go-like interface
(io.Writers and io.Readers!)