Add an option to open the pants server ui.

This also fixes a bug in rcfile augmentation of args.

Testing Done:
$ tail -4 ~/.pants.rc
[server]
options:
  --server-open

$ PANTS_DEV=1 ./pants goal server
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pantsbuild-pants/src/python/pants/bin/pants_exe.py ***
(using pantsrc expansion: pants goal server --server-open)
Server already running at http://localhost:50579
Created new window in existing browser session.

$ PANTS_DEV=1 ./pants goal killserver server
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pantsbuild-pants/src/python/pants/bin/pants_exe.py ***
(using pantsrc expansion: pants goal killserver server --server-open)
Killed server with pid 26443 at http://localhost:50579
Launching server with pid 27492 at http://localhost:36291
Created new window in existing browser session.

Bugs closed: 165

Reviewed at https://rbcommons.com/s/twitter/r/418/