Add option to write out makefile with specific version info

I saw #567000: drush create makefile or something similar but this is different. Packaged install profiles on d.o are going to require specific versions, not just the project names or project => major version mappings. However, we're going to support packaging from profilename-drupalorg.make for what's on d.o, and leave profilename.make for people using the profile directly perhaps the maintainer). If profilename-drupalorg.make doesn't exist, we'll look for profilename.make.

One obvious workflow would be to just have project names in profilename.make, but the maintainer could run something like:

drush make profilename.make --write-makefile=profilename-drupalorg.make [other options]

which would just write out a makefile with the specific versions of exactly what it downloaded automatically. That'd significantly ease the job for d.o profile maintainers so they get the best of both worlds: automated building of the recommended releases of everything they're using, but totally reproducible packaged releases of their profile.

Seems like it'd be extremely easy to write, no? If no one's implemented this by the time I'm done with the rest of my list, I'll taken a stab at it. ;)