Added support for custom integration file loading

[coswat]

This is a small change ( non breaking ), that'll let us add integration files from our local dir.

Inside base46

integrations = {
  "dap", 
  "custom",  -- looks inside our specified dir
   -- in this case its lua/int/custom.lua
  "anyelse", -- lua/int/anyelse.lua
}, 
integrations_dir = "int" 

Why this change? Because it's a lot easier to test it out and develop new integrations files for base46.