Use fork ggrossman/telnetjs instead of baking in modified telnet.js

Use forked `wez-telnet` at https://github.com/ggrossman/telnetjs. The fork lets us negotiate options more flexibly and exposes telnet protocol constants for us to use. Hopefully, the wez-telnet author will merge the changes.

- Deleted baked-in copy of `telnet.js`
- Added dependency on https://github.com/ggrossman/telnetjs to `package.json`
- Add `do` event handler to handle echo/SGA negotiation instead of baking it into a modified copy of telnet.js
- Delete telnet protocol constants like `OPT_ECHO` from `cb_server.js`; use the ones exported by the new `telnet.js`