Add special keys for backspace, delete and F1-F12

I started working on a packer template for OmniOS and needed F2 and backspace to navigate the installer. I added the additional function keys and delete in the interest of completeness. I considered shortening "backspace" to "bs" and "delete" to "del", but opted for verbosity. @mitchellh, if you'd prefer shorter versions, a la "esc", let me know and I'll update the PR.