show --modules will output one module per line

As suggested by @agladysh in #210

So the output of, say, `luarocks show --modules oauth` will be:

~~~~
OAuth.helpers
OAuth
OAuth.coreLuaNode
OAuth.coreLuaSocket
~~~~

instead of 

~~~~
OAuth.helpers OAuth OAuth.coreLuaNode OAuth.coreLuaSocket
~~~~
Use \n as separator to keys_as_string
Merge pull request #223 from ignacio/patch-2

show --modules will output one module per line