resize pty window at startup and when we get a SIGWINCH
resize pty window at startup and when we get a SIGWINCH

- fixes #32
- thanks again to @nelhage. i would never have figured this out without
  https://github.com/nelhage/reptyr as an example to look at.

ninjudd/drip,nriyer/drip,mjwall/drip,ninjudd/drip,mjwall/drip,mjwall/drip,nriyer/drip,nriyer/drip,ninjudd/drip