Automatically make LuaRocks modules available in awesome (#1999)

Running eval $(luarocks path --bin) before running awesome is not all
that easy. It's a lot easier to do the equivalent for this in rc.lua.
Plus, if LuaRocks is not installed, this will silently do nothing.

Signed-off-by: Uli Schlachter <psychon@znc.in>

awesomerc.lua

@@ -1,3 +1,7 @@
+-- If LuaRocks is installed, make sure that packages installed through it are
+-- found (e.g. lgi). If LuaRocks is not installed, do nothing.
+pcall(require, "luarocks.loader")
+
 -- @DOC_REQUIRE_SECTION@
 -- Standard awesome library
 local gears = require("gears")