Improve plugin protocol about go-script-bash

[mbland]

#126, #128, #130, #135, and #136 have effectively implemented the new protocol, which is very similar to npm's node_modules schema. Plugins can be nested arbitrarily deep, with command script, module, and plugin lookup paths scoped correctly at every level in the hierarchy.

Next steps would be to document the protocol and write a tool to automatically install plugins. But the semantics of the protocol are themselves done, so this issue is closed and I'll file new ones.