Only source ssh-agent-setup.sh on Darwin.
Mac adds the id_rsa password to the keychain, so we just need to foward that agent on.
Merge branch 'master' of https://github.com/l1m5/dotfiles

* 'master' of https://github.com/l1m5/dotfiles:
  Paths passed to sed need to be escaped. Didn't notice this before. This caused problems with set-pond and set-rtwd
  Certain versions of mktemp require 3XXX. My version on Mac didn't require this, but Linux does.
  Make sure we use the dark background style in vim.