update help text for dea-ads
add 'cf shell' command

launches a Pry session in `cf`'s context. example:

  tools-cf-plugin(master): bundle exec cf shell
  [1] pry(#<CFTools::Shell>)> client.apps
  => [#<CFoundry::V2::App '046ff7b4-2bfe-4156-85a6-53ed2f8e056a'>]
  [2] pry(#<CFTools::Shell>)> invoke :apps
  Getting applications in alex... OK

  name        status    usage      url
  hello-rbx   stopped   1 x 256M   hello-rbx.a1-app.cf-app.com
  => [["\e[1mname\e[0m", "\e[1mstatus\e[0m", "\e[1musage\e[0m", "\e[1murl\e[0m"],
  ["\e[36mhello-rbx\e[0m",
    "\e[33mstopped\e[0m",
    "1 x 256M",
    "hello-rbx.a1-app.cf-app.com"]]
  [3] pry(#<CFTools::Shell>)>