Option to just download

[daurnimator]

I'd like it if hererocks just had an option to download (and pre-fill the cache) and skip the build/install step.

[mpeterv]

Okay, so let's say, if --download is passed, location argument can be used as a place to unpack sources for Lua/LuaRocks instead of being used as the installation prefix. I guess then it should not allow downloading Lua/LuaJIT and Luarocks at the same time.

[daurnimator]

I didn't even want it to unpack: just pre-fill the cache for operations at a later date (e.g. if lua.org/github are down).