New bin/doom (eventual replacement for make)

This commit adds bin/doom, which acts as the middle man that make once
was (and will stay for a while, though the documentation will shift away
from using it). It does everything the previous make interface did, but
is faster and more flexible. bin/doom should eventually replace the
makefile.

bin/doom also makes it easier to run Doom outside of ~/.emacs.d and
~/.doom.d with, for example:

  bin/doom run -p ~/.other.doom.d/ -e ~/.other.emacs.d

bin/doom.cmd is included for Windows users, but I don't recommend using
it yet. It hasn't been tested nor have I ever written a batch script
before.

Also update init.example.el with new defaults.

bin/doom.cmd

@@ -0,0 +1,14 @@
+:: Forward the ./doom script to Emacs
+
+@ECHO OFF
+PUSHD "%~dp0" >NUL
+
+IF %1=="run" (
+   SHIFT
+   emacs -Q $* -l init.el -f "doom|run-all-startup-hooks"
+) ELSE (
+   emacs --quick --script ./doom -- $*
+)
+
+POPD >NUL
+ECHO ON