Allow stacks to control the command passed to spk init

[zenhack]

The Go stack I submitted the other day just has this in launcher.sh:

exec /opt/app/app

This level of indirection is pointless, and it also results in the shell being pulled into the spk, despite (often) not being needed for anything else. Ideally, there would be some way for a stack to specify the command passed to spk init, so we could just invoke /opt/app/app directly, bypassing the shell.

[paulproteus]

The difficulty here is that it would require modifying the call to "spk init", though I guess that's not *that* hard. It does mean that the go stack would deviate from the docs slightly, which is arguably too bad. I almost wish that it would make a launcher.sh which contained just #!/opt/app/app and therefore no shell was needed, I *think*. Oh, but then again, the way we call "spk init" does pass launcher.sh to bash, to avoid the situation for Windows where launcher.sh isn't marked executable. Well, I guess those are the technical issues. Design ideas or PRs are very welcome!