Added a test related to expanding folders
Added config file

Config file is a Lua file with several assignments to global variables.

E.g.
   compat = true
   globals = {"-", "my_special_global"}
   unused_args = false

By default, luacheck CLI traverses directories from current working directory up to / looking for .luacheckrc
Lookup can be disabled via --no-config flag.
Path to config can be explicitly set via --config=<path> option.

TODO: write tests, test on Windows