Add a configuration option to specify manually installed rocks (#177)

The patch adds a new configuration option - cfg.rocks_provided.
This option contains a list of packages that are either builtin or
manually installed outside of LuaRocks package management system:

    rocks_provided = {
      [module_name] = module_version.."-"..rockspec_version,
      -- ...
    }

These settings are taken into account during dependency resolution
and when search and install commands are used. An attempt to install
a rock from the list causes an error.

LuaRocks automatically adds following modules to the list:

    rocks_provided = {
       lua = lua_version.."-1",

       -- For Lua 5.2 or later:
       bit32 = lua_version.."-1",

       -- For LuaJIT:
       luajit = lj_version.."-1",
       jit = lj_version.."-1",
       ffi = lj_version.."-1",
       bit = lj_version.."-1",
       luabitop = lj_version.."-1"
    }

where lua_version extracted from _VERSION (e.g. 5.2),
      lj_version extracted from jit.version (e.g. 2.0.2).

Built-in values can be overridden by configuration options.

Closes #177.