Accepts custom Hammerspoon config dirs in helper

[lkubb]

This removes the hardcoded Hammerspoon config path from the init.lua helper and replaces it with hs.configdir.

I have been using this small modification to the init.lua helper for the last hour. It works well with my custom config directory and thus fixes https://github.com/AdamWagner/stackline/issues/96. From what I can tell, it should not interfere with anything else.

Just wanted to give back something. This is my first contact with Lua, Hammerspoon and Stackline, so feel free to comment/modify/reject. Cheers!