`brew sh`: a shell with superenv loaded

If you need to build software on OS X by hand, a brew shell is the way to do it.

Closes Homebrew/homebrew#14381.

Library/Homebrew/cmd/sh.rb

+require 'superenv'
+require 'formula'
+
+module Homebrew extend self
+  def sh
+    if superenv?
+      ENV.x11 = true
+      ENV.deps = Formula.installed.select{|f| f.keg_only? and f.opt_prefix.directory? }.map(&:name)
+    end
+    ENV.setup_build_environment
+    ENV['PS1'] = 'brew \[\033[1;32m\]\w\[\033[0m\]$ '
+    puts <<-EOS.undent_________________________________________________________72
+         Your shell has been configured to use Homebrew's build environment: this
+         should help you build stuff. Notably though, the system versions of gem
+         and pip will ignore our configuration and insist on using the
+         environment they were built under (mostly). Sadly, scons will also
+         ignore our configuration.
+         When done, type `exit'.
+         EOS
+    exec ENV['SHELL']
+  end
+end