[core.lib] Add basic "pretty" printing to `store_conf'.

[eugeneia]

The format is as follows:

{
  array = {
    1,
    2,
  },
  table = {
    foo = 1,
    bar = 2,
  },
}

Its not really pretty, but somewhat readable and the algorithm is simple. Idiomatic Lua formatting would be much more complicated as far as I can imagine.

[lukego]

Great! Could you please rebase?