Add support for $CUSTOM_BUILD_SCRIPT.

If this env variable is set, the named script from package.json/scripts will
be run before collecting the slug.

Example:

  Given package.json:

  {
    scripts: {
      build_for_heroku: "gulp"
    }
  }

  And env setup:

  heroku config:set CUSTOM_BUILD_SCRIPT=build_for_heroku

  Then this will be run:

  npm run build_for_heroku