Late-stage .lateslugignore removal of files before slug compilation (soft recreation of rolled back heroku change 179)

Unfortunately, the .slugignore file is processed before the asset precompile stage of the buildpack.  This was changed (https://devcenter.heroku.com/changelog-items/179) and then pretty quickly rolled back (https://devcenter.heroku.com/changelog-items/185).  

The original goal still seems quite sound.  So, recognizing the global adjustment is rough, I've gone after the next logical step back by adjusting the buildpack.  My implementation relies on a an optional .lateslugignore file and if the buildpack finds the file it will act on the extensions found, or just keep moving quietly along (i.e., new functionality at little cost to legacy users)..  

The benefit here is primarily in much smaller slug sizes and streamlined deployments.  This works for me, but feel free to reject/accept this -- I don't want to take and change this work without giving back, at least a little.