Comprehensive xterm-style keybindings for Emacs users

These are the escape sequences/keybindings that Emacs expects to see
from an xterm.  Import them into iTerm as a new kind of preset, so avid
Emacs users can use all the Ctrl+Meta+Shift+whatever keys they're used
to having in X11 or OSX Emacs.  (org-mode users in particular might find
this helpful.)

I pulled these from Emacs's term/xterm.el mappings programmatically, and
I've been using them myself for a few days, so I'm confident most of the
more-commonly used ones are correct.  They also work well with tmux
(although I've had a couple minor issues with zsh and
Option+left/right).

Note, however, that these keybindings trample on some of the default
global iTerm keybindings (just Shift+PgUp/PgDn AFAIK).  I've left those
in for now--personally I prefer Cmd+... for keys handled by iTerm
itself--but I can filter them out in a subsequent commit, if desired.